C cathodes for Li-ion batteriesï¼First

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Mn-doped Fe1-xMnxF3•0.33H2O/C cathodes for Li-ion batteries#First-principles calculations and experimental study Jing Ding, Xiangyang Zhou, Hui Wang, Juan Yang, Yuning Gao, and Jingjing Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17069 • Publication Date (Web): 02 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

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ACS Applied Materials & Interfaces

Mn-doped Fe1-xMnxF3·0.33H2O/C cathodes for Li-ion batteries： First-principles calculations and experimental study Jing Ding, Xiangyang Zhou, Hui Wang, Juan Yang, Yuning Gao, Jingjing Tang* School of Metallurgy and Environment, Central South University, Changsha 410083, China

*Corresponding author: Jingjing Tang E-mail address: [email protected] Tel: +86-731-88836329; Fax: +86-731-88871017

KEYWORDS: Li-ion batteries, Cathode material, Mn-doping, First-principles calculations, Fe1-xMnxF3·0.33H2O/C nanocomposites

ABSTRACT Increasing attention has been paid on iron fluoride as an alternative cathode material for Li-ion batteries owing to its high energy density and low-cost. However, the poor electric conductivity and low diffusivity for Li-ions set great challenge for iron fluoride to be used in practical LIBs. Here, we employ first-principles calculations to probe the influence of Mn-doping on the crystal structure and electronic structure of FeF3·0.33H2O. The calculated results suggest that Mn-doping can enlarge the hexagonal cavity and reduce band gap of FeF3·0.33H2O as well as improve its intrinsic conductivity. Furthermore, Fe1-xMnxF3·0.33H2O/C (x = 0, 0.06, 0.08, 0.10) nanocomposites were successfully fabricated by hydrothermal method and ball-milling. Attributing to the Mn-doping effect combining high conductive acetylene black (AB) modification, the typical Fe0.92Mn0.08F3·0.33H2O/C composite exhibits a high discharge capacity of 180 mAh g-1 at 50 mA g-1 after 100 cycles and delivers excellent cycling stability as well as good rate capability.

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INTRODUCTION Nowadays Li-ion batteries (LIBs) are receiving increasing attentions in a wide range of applications from portable electronics to transportation (such as hybrid electric vehicle and pure electric vehicles), for their advantages in efficiency, no memory effect and environmental benignity, etc

1-3.

Although several breakthroughs

have been made in the last decades, the reversible specific capacities of commercial cathode materials (e.g. LiCoO2, LiFePO4) are still too low to satisfy ever-growing demands for advanced LIBs with high energy/power density

4-5.

Fortunately,

transition metal fluorides on the basis of the conversion reaction have triggered worldwide interest as alternative cathode materials for LIBs application due to their relatively high voltage plateau and large theoretical capacities

6-7.

For instance, CuF2

has an working potential of 3.55 V and a theoretical capacity of 528 mAh g-1 8-9, FeF3 (2.74 V, 712 mAh g-1)

10-11,

FeF2 (2.66 V, 571 mAh g-1)

12-13

etc. Among fluoride

based cathodes, iron trifluoride, appears to be unique as a potential candidate cathode owing to its high theoretical capacity, high operating voltage, environmentally friendly and low-cost,

14-15.

As a polymorph of iron fluoride, FeF3·0.33H2O has a

unique hexagonal cavity composed of six octahedrons by corner-sharing, where H2O molecules exist in the center of cavities, which are suitable for intercalation and deintercalation of Li-ions. In addition, the existence of crystal water can stabilize the structure and decrease band gap as well as enhance the electronic conductivity. Therefore, these advantages make it a promising cathode material for LIBs 16-17. In the charge-discharge process, it undergoes two continuous insertion and conversion reactions. During the initial intercalation of Li-ions, the intercalated LiFeF3·0.33H2O is produced (225 mAh g-1, 1e- transfer). With further discharge to below 2.0 V, LiFeF3·0.33H2O is fully destroyed to form LiF and Fe·0.33H2O through a conversion reaction (450 mAh g-1, 2e- transfer)18. FeF3  0.33H 2O  Li  e-  LiFeF3  0.33H 2O

(1)

LiFeF3  0.33H 2O  2Li  2e-  3LiF  Fe  0.33H 2O

(2)

Nevertheless, it still faces a number of obstacles such as poor ionic and low electronic conductivities owing to the ionic nature of Fe-F bonds, which result in the fast capacity fade and low experimental capacity, thus inhibiting its practical application in LIBs

19.

To address this intrinsic drawback, many works aiming at

improving the electrochemical property of FeF3·0.33H2O have been conducted by


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adding conductive materials and modifying conductive layer on its surface. Fan et al. 20

has demonstrated that the hybrid with carbon nanohorns can effectively enhance the

electrochemical

performance

of

FeF3·0.33H2O.

Li

et

al.

21

fabricated

FeF3·0.33H2O/graphene hybrid using a tactful ionic-liquid-assisted method, and it maintained 74 and 115 mAh g-1 at 8 and 2 A g-1 after 250 cycles, respectively. Li et al. 22

modified FeF3·0.33H2O with single-wall carbon nanotube, which showed

outstanding electrochemical features. Recently, FeF3·0.33H2O nanoparticles were packaged into three-dimensional order mesoporous carbons to relieve volume effect and improve the electronic conductivity

17.

Furthermore, mixing FeF3·0.33H2O with

AB via ball-milling is another simple and effective route to improve electronic conductivity. Wei et al.

19

prepared FeF3·0.33H2O/C nanocomposite utilizing a

one-pot chemical approach and ball-milling method, which exhibited a high first discharge capacity of 276.4 mAh g-1 and retained 193.5mAh g-1 after 50 cycles at 237 mA g-1 in the region of 1.5-4.5 V. In addition, nanosized TiO2-coated FeF3·0.33H2O exhibited outstanding cycling stability and remarkable rate capability 23. Although the addition of conductive materials and surface modification can both facilitate the charge transfer and Li-ions diffusion ability as well as maintain the structural stability to some extent, the intrinsic electronic conductivity and the Li-ion mobility of bulk phase are still too low. In addition to hybriding FeF3·0.33H2O with conductive matrix, the doping of anions and high valence transition metal cations was demonstrated to be an effective method to relieve the intrinsic drawbacks of iron fluoride

24-26.

Since the

ionic radius of manganese and iron are very close (0.650 Å of Mn3+ Vs 0.645 Å of Fe3+) 27, meanwhile the multivalent feature of Mn makes it a promising candidate as effective doping element for improving the conductivity of FeF3·0.33H2O. However, to the best of our knowledge, the modification of FeF3·0.33H2O by Mn doping as cathode material has been rarely reported. Accordingly, it is of great necessary to probe and reveal the effects of Mn doping on the electronic structure and electrochemical performance of FeF3·0.33H2O. In this work, the effects of Mn-doping on the crystal structure and electronic structure of FeF3·0.33H2O were identified via first-principles calculations to have a better understanding on the mechanism of band gap reduction, providing crucial theoretical guidance of rational design of cathode material in experiment. Then, serial Fe1-xMnxF3·0.33H2O (x = 0, 0.06, 0.08, 0.10) cathode materials for LIBs were successfully fabricated by hydrothermal method to verify the above calculations.



Page 4 of 21

Furthermore, the obtained Fe1-xMnxF3·0.33H2O was composited with AB by ball-milling to improve the electronic transmission capability among particles. The typical Fe0.92Mn0.08F3·0.33H2O/C shows a high reversible specific capacity of 180 mAh g-1 at 50 mA g-1 after 100 cycles and remarkable rate performance with high discharge capacities of 450.2, 269.6, 210.9, 158.3 and 103.8 mAh g-1 at current densities of 50, 100, 250, 500 and 1000 mA g-1, respectively. Therefore, the fabricated Fe1-xMnxF3·0.33H2O/C cathode materials with Mn-doping in crystal structure and further high conductive AB modification exhibit significant prospect in the high performance metal fluoride cathode of LIBs.

EXPERIMENTAL AND COMPUTATIONAL SECTION Preparation of materials The preparation of Fe1-xMnxF3·0.33H2O/C (x = 0, 0.06, 0.08, 0.10) nanocomposites is schematically depicted in Scheme 1. At first, Iron(III) chloride hexahydrate (FeCl3·6H2O, 0.1mol) was dissolved in deionized water (100 mL), then sodium hydroxide (NaOH, 0.33mol) was added with stirring for 1h and aged for 12 h. The Fe(OH)3 precipitations were collected via filtration and washed for several times, accompanied by drying at 80 °C for 12 h in air. Then, MnO2 and Fe(OH)3 in a molar ratio of 0:1.0, 0.06:0.94, 0.08:0.92 and 0.10:0.90 were dissolved in 40 ml deionized water in a Teflon container and constant stirring for 1 h. Subsequently, 30mL HF aqueous solution (40 wt.%) was cautiously poured into the above mixed solution, and the mixture was continuously stirred for 12 h at 80 °C, followed by heating at 80 °C to remove the excess water and HF. Afterwards, the residue was dried at 80 °C to acquire the Fe1-xMnxF3·3H2O precursor, and the obtained precursor was annealed at 220 °C for 10 h in tube furnace under Ar atmosphere to remove the hydration water for obtaining the Fe1-xMnxF3·0.33H2O products. To further improve the conductivity of the as-prepared Fe1-xMnxF3·0.33H2O, AB, was added (weight ratio of Fe1-xMnxF3·0.33H2O : AB = 85 : 15). Finally, Fe1-xMnxF3·0.33H2O/C nanocomposites were obtained by ball-milling in air at 500 rpm for 4 h, and further dried at 200 °C for 3 h in Ar atmosphere. Computational Method The present calculations were implemented with the Vienna Ab-initio Simulation Package (VASP)

28-29.

The projector-augmented-waves (PAW)

30-31

technique and

generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) 32 were utilized to address core-electron interaction and exchange-correlation potential,


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respectively. The cutoff energy was set to 520 eV and 4 × 2 × 4 generated with the Monkhorst-Pack scheme 33. To deal with Coulomb interactions of d-electrons of Mn and Fe in the Mn-doped FeF3·0.33H2O, the GGA+U method Within the GGA+U approach, an Ueff = 5 eV

24

and Ueff = 4 eV

34

was carried out.

35

were used for Fe

atom and Mn atom, respectively. In addition, the behavior of water in present calculation was accurately depicted by optB88-vdW

36-37

calculations within VASP.

Of all calculations, spin-polarized calculation was implemented. Materials characterizations The structures of synthesized materials were recorded by X-ray diffraction (XRD, Rigaku-TTRIII) using Cu Ka radiation. The morphologies of the samples were characterized by scanning electron microscope (SEM, Nova NanoSEM230, USA). TEM measurements and selected-area electron diffraction (SAED) were acquired by a transmission electron microscope (JEOL JSM-2100F). Electrochemical tests The electrochemical properties of as-prepared nanocomposites were studied in CR2025 coin-type cell with Li metal as anode. The composite electrodes were fabricated via mixing AB, polyvinylidene fluoride (PVDF) and the active material Fe1-xMnxF3·0.33H2O/C nanocomposites (or FeF3·0.33H2O, Fe0.92Mn0.08F3·0.33H2O) with a mass ratio of 10 : 10 : 80 in NMP as solvent. The slurry was coated onto Al foil, dried at 120 °C for 12 h in vacuum drying oven and then tailored a circular disk (d = 1.0 cm, loading density ca. 1-1.3 mg cm-2). The electrodes were assembled in an Ar-filled glove box (Super 1220/750, Shanghai Mikrouna Co. Ltd.), with a porous polypropylene membrane (Celgard 2400) as the separator, and 1 M LiPF6 solution in ethylene carbonate/ethylene methyl carbonate/dimethyl carbonate (EC/EMC/DMC, 1:1:1 vol.%) as the electrolyte. Galvanostatic charge and discharge performances were measured by a Land battery tester (Land CT2001A, Wuhan, China) between 1.5 and 4.5 V at ambient temperature after all the cells had been aged for 12 h. The specific capacity is calculated based on Fe1-xMnxF3·0.33H2O. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were measured on an electrochemical workstation (P4000, PARSTAT MC). EIS was performed with the frequency range from 10 mHz to 100 KHz. Moreover, CV curves were obtained at a scanning rate of 0.2 mV s-1 between 1.5 and 4.5 V.

RESULTS AND DISCUSSION Structural properties



Page 6 of 21

The structure of FeF3·0.33H2O belongs to orthorhombic structure. The structure cell of the FeF3·0.33H2O includes 12 formula units. Fe12F36·4H2O and Fe11MnF36·4H2O were regarded as FeF3·0.33H2O and Fe0.92Mn0.08F3·0.33H2O, respectively. Three different structures of one Mn replacement on the Fe sites are taken

into

account

and

shown

in

Figure

S1.

Fe0.92Mn0.08F3·0.33H2O(I),

Fe0.92Mn0.08F3·0.33H2O(II) and Fe0.92Mn0.08F3·0.33H2O(III) are labeled, respectively. To validate the site preference of the Mn doped FeF3·0.33H2O, the total energies of Fe0.92Mn0.08F3·0.33H2O(I), Fe0.92Mn0.08F3·0.33H2O(II) and Fe0.92Mn0.08F3·0.33H2O(III) were calculated corresponding to -188.062 eV, -188.145 eV and -188.146 eV, respectively. It can be found that the three configurations have close total energy and Fe0.92Mn0.08F3·0.33H2O(III) is the most stable structure because of the lowest total energy. To understand the influence of Mn-doping on the structural properties of FeF3·0.33H2O, the lattice parameters of pure FeF3·0.33H2O (experiment38 and present work) and Fe0.92Mn0.08F3·0.33H2O(III) are compared and listed in Table 1. Compared with FeF3·0.33H2O, the crystal volume of Fe0.92Mn0.08F3·0.33H2O(III) become larger because the radius of the Mn3+ (0.650 Å) is slightly bigger than that of the Fe3+ (0.645 Å). Moreover, it is noticed in Figure S2 that the hexagonal cavity of FeF3·0.33H2O is smaller than that of Fe0.92Mn0.08F3·0.33H2O(III), which indicates that the hexagonal cavity of FeF3·0.33H2O can be enlarged by Mn-doping, thus benefiting Li-ions diffusion during charge-discharge process. Electronic structural properties and bonding mechanism The

spin-polarized

band

structure

of

pure

FeF3·0.33H2O

and

Fe0.92Mn0.08F3·0.33H2O(III), combined with the corresponding density of states (DOS) are presented in Figure 1. The band gap of FeF3·0.33H2O (Figure 1a) is 0.99eV, and it matches well with the theoretical calculated value (0.95 eV) reported by Li et al. 39. After a Mn atom replace the Fe atom in FeF3·0.33H2O, the band gap of Fe0.92Mn0.08F3·0.33H2O(III) (Figure 1b) decrease to 0.78 eV, which indicates that Mn-doping can enhance the electronic conductivity of FeF3·0.33H2O. In the energy range from -4 to 4 eV, the DOS of pure FeF3·0.33H2O and Fe0.92Mn0.08F3·0.33H2O(III) has been carried out to investigate the distinction near Fermi level. For FeF3·0.33H2O (Figure 1c), the peaks below -2.2 eV mostly originate from the contribution of the electrons of F-2p orbits and Fe-3d orbits. The valence bands basically consist of O-2p orbits between -2.2 eV and Fermi level, while the conduction bands are mostly comprised of Fe-3d orbits and a bit F-2p orbits. For the Fe0.92Mn0.08F3·0.33H2O(III)


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(Figure 1d), the peaks between -4 eV and Fermi level exhibit similar features with the main contribution from the Fe-3d orbits and F-2p orbits, but the Mn-3d orbit appears, which causes the spin splitting impurity states and O-2p above the valence band. Hence, the spin down states pass through the Fermi level and reveal half-metallic characteristics, suggesting that the conductivity of FeF3·0.33H2O can be enhanced by introducing Mn atom. For further analyzing the influence of Mn-doping on the electronic structure, the deformation charge densities of pure FeF3·0.33H2O and Fe0.92Mn0.08F3·0.33H2O(III) are calculated and presented in Figure 2a and b, respectively. It is obvious that stronger ionic bond exists between F and Fe atoms, which attributes to the high electronegativity of F atoms and the charge transfer from Fe to F atoms. After Mn atom is doped in FeF3·0.33H2O, F atoms acquire less electrons from the neighbouring Mn atom in the Fe0.92Mn0.08F3·0.33H2O(III). In order to further describe charge transfer quantitatively, the Bader charger analysis on the basis of the AIM theory 40 is carried out. In this approach, it can give more information on the bonding mechanism in detail. As shown in Figure 2c, it can be found that the Fe atoms lose about 2.1|e| and F atoms gain -0.69~0.71|e|, indicating that the charge transfers from Fe to F atoms, which

matches

well

with

the

above

results

(Figure

2a).

For

the

Fe0.92Mn0.08F3·0.33H2O(III) (Figure 2d), it can be seen that approximately 2.0|e| transfers from Mn atom to its neighboring F atoms, which shows that the ionic bond of Mn-F is weaker than Fe-F ionic bond, while the covalency of Mn-F is stronger than that of Fe-F by Mn doping. Therefore, the component of covalent bond in FeF3·0.33H2O increases making the enhancement of conductivity of FeF3·0.33H2O after Mn doping. Structure and morphology characterization Inspired by the improved elctronic conductivity and Li-ions diffusivity of FeF3·0.33H2O after Mn-doping, Fe1-xMnxF3·0.33H2O (x = 0, 0.06, 0.08, 0.10) composites were synthesized via solvothermal method for evaluating their electrochemical performance as cathodes for LIBs. Furthermore, AB is used as conductive coating material to promote charge transport within electrodes.

Figure

3a shows the XRD patterns of Fe1-xMnxF3·0.33H2O (x = 0, 0.06, 0.08, 0.10). It is found that all samples exhibit similar diffraction features, which matches well with the standard peaks of FeF3·0.33H2O (PDF No. 76-1262). Obviously, there is no extra peaks detected in the Mn-doped samples, demonstrating that Mn atom has inserted



Page 8 of 21

into the crystal structure of FeF3·0.33H2O rather than forming other new crystal phases. As seen in Figure 3b, the distinct diffraction peak positioned at around 2θ = 13.8° corresponding to (1 1 0) diffraction peak shifts slightly towards a smaller angle with the Mn doping concentration increases, which should be attributed to the bigger radius of Mn3+ compared with Fe3+. The lattice parameters (b and c) of Mn doped FeF3·0.33H2O in Table 1 are larger than those of pure FeF3·0.33H2O, but intrinsic crystal structure is not changed basically. According to the Eq. (1), the interplanar crystal spacing d(1 1 0) can be calculated owing to their orthorhombic structure. The interplanar crystal spacing of pure FeF3·0.33H2O is 6.431 Å, while the value for Mn doped FeF3·0.33H2O turn out to be 6.434 Å. Based on the Bragg’s equation (2d·sinθ = nλ), the enlarged interplanar distance will bring about reducing θ value, which makes (1 1 0) diffraction peak shifting slightly to a smaller angle. d (hkl ) 

The

morphologies

1 2

2

h k  l    +  +  a b c

of

(3)

2

FeF3·0.33H2O

and

Fe0.92Mn0.08F3·0.33H2O

are

characterized by SEM and TEM, as showed in Figure 4a-c. Besides, the SEM images of Fe0.94Mn0.06F3·0.33H2O and Fe0.9Mn0.1F3·0.33H2O are shown in Figure S3a and S3b. All samples are made of rectangular solid with the particle sizes of 3-15 μm. And the small particles adhere to the surface of the rectangular particles increased after Mn-doping, which can benefit the electrolyte penetration and enhance electrochemical properties

26, 41-42.

It is well known that the smaller particle sizes of

Fe1-xMnxF3·0.33H2O materials can increase the specific surface areas and shorten effectively ion diffusion distances

43.

In addition, the HRTEM image in Figure 4d

indicates a highly crystallinity of Fe0.92Mn0.08F3·0.33H2O. The lattice fringe with average distance of 0.325 and 0.270 nm marked by yellow arrow match well with the (2 2 0) and (2 0 2) crystal planes of FeF3·0.33H2O, respectively. Furthermore, the corresponding SAED pattern (insert Figure 4d) suggests that the product corresponds to FeF3·0.33H2O 44, which further proves the XRD results (Figure 3). To further confirm the element distribution and composition of the Fe0.92Mn0.08F3·0.33H2O, the corresponding EDXs mapping is presented in Figure 4e. Obviously, the F, O, Fe and Mn are evenly dispersed in the interior space of the Fe0.92Mn0.08F3·0.33H2O sample without phase separation. Additionally, the EDX spectrum (Figure 4f) indicates that the sample contains O, F, Fe and Mn, while the


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Cu peaks is caused by the adopted copper collector. As a result, the Fe0.92Mn0.08F3·0.33H2O sample is well fabricated by the facile hydrothermal method. To further enhance the electronic conductivity, Fe1-xMnxF3·0.33H2O (x = 0, 0.06, 0.08, 0.10) is ball-milling with AB and the corresponding SEM images are presented in Figure S3c-f. Obviously, all samples have been varied from cuboid to particles with irregular shape because of agglomeration. In addition, the particle sizes of the Fe1-xMnxF3·0.33H2O/C nanocomposites are decreased to nano scale (50-500 nm) and Fe0.92Mn0.08F3·0.33H2O/C nanocomposite exhibits the most uniform particle size distribution among them. The AB, distributed evenly on the surface of Fe1-xMnxF3·0.33H2O, is also favorable for enhancing its conductivity, and thus benefit improving the electrochemical performances greatly. Electrochemical performances analysis The first-principle calculations results demonstrate that Mn-doping can enhance the conductivity of FeF3·0.33H2O, to further evaluate its effects on the electrochemical

performances

as

cathodes

for

LIBs,

FeF3·0.33H2O

and

Fe0.92Mn0.08F3·0.33H2O were firstly synthesized by solvothermal method. The corresponding cycling performances at 50 mA g-1 in the range of 1.5-4.5 V were tested and showed in Figure 5a. The FeF3·0.33H2O material shows a low initial discharge capacity of 296.4 mAh g-1 with a Coulombic Efficiency (CE) of 93.8% and retains a reversible capacity of 36.2 mAh g-1 after 100 cycles. After Mn-doping, the first discharge capacity of Fe0.92Mn0.08F3·0.33H2O electrode can obtain as high as 384.3 mAh g-1 with a CE of 94.7% and still remains 81.1 mAh g-1 after 100 cycles, which is ascribed to its increased electronic conductivity by introducing Mn atom. When evaluated as cathode materials for LIBs, the reversible capacity of Fe0.92Mn0.08F3·0.33H2O is still too low and further improvement is needed. After ball-milling

with

AB,

the

cycling

performances

of

FeF3·0.33H2O

and

Fe0.92Mn0.08F3·0.33H2O are greatly improved and presented in Figure 5a. As being seen, the FeF3·0.33H2O displays a high initial discharge capacity of 351.9 mAh g-1 with a CE of 87.5% and the reversible capacity of 113 mAh g-1 can be maintained after 100 cycles for FeF3·0.33H2O/C material. The reduced CE (from 93.8% for FeF3·0.33H2O to 87.5% for FeF3·0.33H2O/C) may be attributed to the detrimental reactions 45-46 on the more interface of electrolyte with FeF3·0.33H2O after its particle decreasing. And the Fe0.92Mn0.08F3·0.33H2O/C material obtains an initial discharge capacity of as high as 452.6 mAh g-1 and a reversible capacity of 180 mAh g-1 after



Page 10 of 21

100 cycles, indicating that the high conductive AB can improve the cycling stability greatly.

Besides,

to

further

probe

the

effect

of

Mn-doping,

the

Fe0.94Mn0.06F3·0.33H2O/C and Fe0.9Mn0.1F3·0.33H2O/C samples were prepared and their cycling performances were presented in Figure S4. Similarly, the cycling stability of the Mn-doped FeF3·0.33H2O/C materials has been improved. The reversible capacities of Fe1-xMnxF3·0.33H2O/C (x = 0.06, 0.10) are 145.6 and 151.2

mAh

g-1

after

100

cycles,

respectively.

Interestingly,

the

Fe0.92Mn0.08F3·0.33H2O/C sample delivers the highest discharge capacity among these materials,

which

is

not

a

coincidence.

The

average

grain

size

of

Fe1-xMnxF3·0.33H2O/C is listed in Table S1. As observed, the Fe1-xMnxF3·0.33H2O/C composites show increased gain size comparied with undoped FeF3·0.33H2O/C composites. Although the larger particle will reduce the interface of cathode material contacting with electrolytes and restrain the side reactions, the overgrowth of particle can restrict Li+ intercalation and deintercalation 43 (see Figure 7), leading to the bad cycling performances. In addition, the TEM, HRTEM and SAED images of the Fe0.92Mn0.08F3·0.33H2O/C full charged to 4.5 V after 25 cycled are depicted in Figure S6. The detected lattice stripe corresponding to (2 2 0) plane of FeF3·0.33H2O, indicate structural stability of Fe0.92Mn0.08F3·0.33H2O/C after cycles, implying its good reversibility. Therefore, the above analysis reveals that the moderate Mn-doping (x = 0.08) combining with high conductive AB modification is beneficial to enhance the cycling performance of FeF3·0.33H2O cathode. The charge-discharge profiles of FeF3·0.33H2O/C and Fe0.92Mn0.08F3·0.33H2O/C nanocomposites after first, 10th, 20th, 40th and 100th cycles are shown in Figure 5b and c. Two voltage plateaus in the initial discharge curve appear at ~2.9 and ~1.5 V are caused by an intercalation reaction and a conversion reaction

18.

The

electrochemical reaction mechanism of FeF3·0.33H2O with Li was further studied (see

Supporting

Information).

Furthermore,

the

Fe0.92Mn0.08F3·0.33H2O/C

nanocomposite shows the lower charge voltage plateau and higher discharge voltage plateau from first to 100th cycles, revealing that it possess a smaller electrochemical polarization and thus can alleviate the voltage hysteresis. In order to explore the enhancement of electrochemical performances after Mn-doping, the CV tests of Fe1-xMnxF3·0.33H2O/C (x = 0, 0.06, 0.08, 0.10) in second cycle are performed at a scan rate of 0.2 mV s-1 in the region of 1.5-4.5 V, as illustrated in Figure 5d. It is observed that all samples show similarities, which


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include two pair of oxidation-reduction peaks. Additionally, the peak area of CV curve for the Fe0.92Mn0.08F3·0.33H2O/C sample is the biggest than others, indicating that the Fe0.92Mn0.08F3·0.33H2O/C sample can provide a highest capacity. The redox peaks of FeF3·0.33H2O/C sample are found at 3.28 and 2.87 V, and the potential interval (ΔEp) is 0.41 V. In comparison to FeF3·0.33H2O/C sample, the Fe1-xMnxF3·0.33H2O/C (x = 0.06, 0.08, 0.10) samples have smaller ΔEp, which correspond to 0.39, 0.38 and 0.32 in turn. In general, the smaller ΔEp reveals a lower electrochemical polarization and a superior cyclic stability

43,

which matches well

with the cycling performances (Figure 5a). The above results prove that Mn-doping can enhance the cycling performances since the decrease of band gap results in the improvement of conductivity for FeF3·0.33H2O/C. The rate performances of the FeF3·0.33H2O/C and Fe0.92Mn0.08F3·0.33H2O/C at various current densities (50-1000 mA g-1) with each rate for 10 cycles are presented in Figure 6a to further understand the influence of Mn-doping on rate performance. As the current densities increase, the discharge capacities of the samples are reduced. The FeF3·0.33H2O/C displays discharge capacities of 304.6, 174.4, 106.9, 42.6 and 10.6 mAh g-1 at 50, 100, 250, 500 and 1000 mA g-1, respectively. The Fe0.92Mn0.08F3·0.33H2O/C, by contrast, shows higher discharge capacities of 450.2, 269.6, 210.9, 158.3 and 103.8 mAh g-1 at 50-1000 mA g-1. When the current density finally returns to 50 mA g-1, the capacity of FeF3·0.33H2O/C turn out to be 181.7 mAh g-1 while that of Fe0.92Mn0.08F3·0.33H2O/C can maintain 234.3 mAh g-1. Apparently, the rate performance of FeF3·0.33H2O/C is greatly enhanced by Mn-doping. To further evaluate the cycling stability of electrode, the cycling performances of FeF3·0.33H2O/C and Fe0.92Mn0.08F3·0.33H2O/C at large current density (200 mA g-1) are illustrated in Figure 6b. And the corresponding charge-discharge curves at particular cycles are presented in Figure 6c and 6d, respectively.

The

discharge

capacities

of

the

FeF3·0.33H2O/C

and

Fe0.92Mn0.08F3·0.33H2O/C are 69.1 and 140.3 mAh g-1 after 200 cycles, respectively. Remarkably, the Fe0.92Mn0.08F3·0.33H2O/C exhibits better cycling stability than that of FeF3·0.33H2O/C. Additionally, the Fe0.92Mn0.08F3·0.33H2O/C electrode shows lower charge voltage plateau and upper discharge voltage plateau from first to 200th cycles. The above results primarily ascribed to three points: (i) the band gap reduction (Figure 1) and (ii) the Li-ions diffusion channel enlargement (Figure S2) as well as (iii) the introduction of high conductive AB, which work synergistically improving



Page 12 of 21

cycling and rate performance after Mn-doping. To explain the intrinsic effects of electrochemical properties for Mn-doping, the electrochemical

impedance

spectroscopy

(EIS)

measurements

for

Fe1-xMnxF3·0.33H2O/C (x = 0, 0.06, 0.08, 0.10) are performed and shown in Figure 7. All samples have the similar features, which are composed of a semicircle in the high frequency region corresponding to the charge-transfer resistance (Rct) representing the reaction kinetics of the electrode reaction and a quasi-straight slope in the low-frequency associating with the diffusion impedance (Zw) in the electrode materials. The Nyquist profiles have fitted and the corresponding equivalent circuit model is described in Figure 7a. The electrolyte resistance (Rs) reflects the total impedance of the electrolyte and electrode material. Table 2 lists the fitted impedance parameters, which match well with the experimental results presented in Figure 7a. Obviously, the Fe0.92Mn0.08F3·0.33H2O/C has the smallest Rs and Rct than others, suggesting that it exhibits the highest electron conductivity and best electrochemical property. To assess the effect of Mn-doping on the Li-ions diffusion, the Li-ions diffusion coefficients (DLi+) can be calculated by Eq. (2) 47:

D Li+ =

R 2T 2 2A 2 n 4 F4 C2 σ 2

(4)

where T is absolute temperature (K), R is gas constant, n is the number of electrons per molecule in the redox reaction, A is the surface area of the electrode, C is the molar concentration of Li+, F is Faraday constant, and σ is Warburg coefficient correlated with Z’: Z'=R s +R ct +σω-1/2

(5)

In Eq. (3), ω is angular frequency at the low frequency region. The σ can be calculated as the slope in Figure 7b. Therefore, the DLi+ is obtained by Eq. (2) and (3) and listed in Table 2. Obviously, DLi+ of the Fe0.92Mn0.08F3·0.33H2O/C is larger than others, demonstrating that it has better diffusion kinetics, which verifies the First-principles calculation results (Figure S2). It can be concluded that the remarkable electrochemical performance of Fe0.92Mn0.08F3·0.33H2O/C sample is ascribed to the synergistic effect of higher conductivity and faster Li+ diffusion ability causing by Mn-doping as well as high conductive AB modification. However, the excessive Mn-dopant content will increase Rct and suppress Li+ diffusion, because the overgrowth of grain size will reduce DLi+ and increase Rct43, which has a worse impact


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than the positive effect of decreased band gap. As a result, a proper Mn-doping content combining with high conductive AB modification can greatly improve the conductivity of cathode material, but also can effectively facilitate Li+ diffusion during cyclic process, and thus enhancing its electrochemical performances.

CONCLUSIONS In this work, the Fe1-xMnxF3·0.33H2O/C cathode materials of LIBs have been successfully fabricated via a facile hydrothermal method and ball-milling process. The theoretical calculation indicates that Mn-doping can reduce band gap of FeF3·0.33H2O and enhance its intrinsic conductivity as well as can enlarge the hexagonal cavity. In the experimental study, for further improving the reaction kinetic in electrodes, high conductive AB is employed to coat Fe1-xMnxF3·0.33H2O. The results demonstrate that the Fe0.92Mn0.08F3·0.33H2O/C sample delivers the best cycling performance and good rate performance, which is ascribed to the increased elctronic conductivity and diffusivity for Li-ions owing to synergistic Mn-doping and AB coating effects. Consequently, this study provides promising guidance to design high performance metal fluorides cathode materials for LIBs. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website Supporting Information Available: first-principles calculations results, additional SEM, XRD and TEM characterization and electrochemical test AUTHOR INFORMATION Corresponding Author *Tel: +86 0731 88836329; Fax: +86 0731 88710171. *E-mail address: [email protected] ORCID Jingjing Tang : 0000-0002-6383-469X



Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Nature Science Foundation of China (Grant no. 51871247) and the Project of Innovation-driven Plan in Central South University.

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32869-32874. 45. Pohl, A.; Faraz, M.; Schroeder, A.; Baunach, M.; Schabel, W.; Guda, A.; Shapovalov, V.; Soldatov, A.; Chakravadhanula, V. S. K.; Kuebel, C.; Witte, R.; Hahn, H.; Diemant, T.; Behm, R. J.; Emerich, H.; Fichtner, M., Development of a water based process for stable conversion cathodes on the basis of FeF3. J. Power Sources 2016, 313, 213-222. 46. Yang, J.; Xu, Z.; Zhou, H.; Tang, J.; Sun, H.; Ding, J.; Zhou, X., A cathode material based on the iron fluoride with an ultra-thin Li3FeF6 protective layer for high-capacity Li-ion batteries. J. Power Sources 2017, 363, 244-250. 47. Gao, F.; Tang, Z., Kinetic behavior of LiFePO4/C cathode material for lithium-ion batteries. Electrochim. Acta 2008, 53 (15), 5071-5075.

Schematic

Scheme 1 The schematic representation of Fe1-xMnxF3·0.33H2O/C nanocomposites preparation.



Figures

Figure 1 Band structure and density of states: (a) and (c) FeF3·0.33H2O, (b) and (d) Fe0.92Mn0.08F3·0.33H2O(III).

Figure 2 Deformation charge density and Bader analysis: (a) and (c) FeF3·0.33H2O, (b) and (d) Fe0.92Mn0.08F3·0.33H2O(III).


Page 18 of 21

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Figure 3 XRD patterns of (a) Fe1-xMnxF3·0.33H2O (x = 0, 0.06, 0.08, 0.10), (b) the expanded (110) XRD peak at around 2θ = 13.8°.

Figure 4 SEM images of (a) FeF3·0.33H2O and (b) Fe0.92Mn0.08F3·0.33H2O; (c) TEM and (d) HRTEM images of Fe0.92Mn0.08F3·0.33H2O, SAED pattern of Fe0.92Mn0.08F3·0.33H2O (inset d), (e)



EDXs mapping images (O, F, Fe and Mn) and corresponding (f) EDXs spectrum.

Figure 5 (a) The cycling performances the Fe1-xMnxF3·0.33H2O (x = 0, 0.08) before and after ball-milling at a current density of 50 mA g-1; (b) The charge-discharge voltage profiles FeF3·0.33H2O/C and (c) Fe0.92Mn0.08F3·0.33H2O/C of 1st to 100th; (d) Cyclic voltammetry curves of the Fe1-xMnxF3·0.33H2O/C (x = 0, 0.06, 0.08, 0.10) in second cycle measured at a scanning rate of 0.2 mV s-1.

Figure 6 (a) The rate capability of FeF3·0.33H2O/C and Fe0.92Mn0.08F3·0.33H2O/C varying rates from 50 mA g-1 to 1000 mA g-1 and (b) the corresponding cycling performance at a large current density of 200 mA g-1 at the range of 1.5-4.5 V; The charge-discharge voltage curves (c) FeF3·0.33H2O/C and (d) Fe0.92Mn0.08F3·0.33H2O/C of 1st to 200th.


Page 20 of 21

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Figure 7 (a) EIS analysis of Fe1-xMnxF3·0.33H2O/C (x = 0, 0.06, 0.08, 0.10); (b) The relationship between Z’ and ω1/2 at low frequency region of Fe1-xMnxF3·0.33H2O/C (x = 0, 0.06, 0.08, 0.10); The equivalent circuit model of EIS (insert a).

Tables Table 1 The calculated lattice constants of pure FeF3·0.33H2O and Fe0.92Mn0.08F3·0.33H2O(III)

Compounds

a (Å)

b(Å)

c(Å)

V(Å3)

FeF3·0.33H2O (experiment 38) 7.423 12.730 7.526 711.17 FeF3·0.33H2O (present work)

7.447 12.752 7.467 712.09

Fe0.92Mn0.08F3·0.33H2O (III)

7.447 12.778 7.487 712.45

Table 2 Values of Rs, Rct and DLi+ for the Fe1-xMnxF3·0.33H2O/C (x = 0, 0.06, 0.08, 0.10) samples.

Fe1-xMnxF3·0.33H2O/C x=0 x = 0.06 x = 0.08 x = 0.1

Rs(Ω) 2.776 1.480 1.221 1.455

Rct(Ω) 270.1 252.6 147.3 193.2

Table of Content


DLi+(m2 s-1) 2.0×10-16 2.9×10-16 3.4×10-16 3.2×10-16

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