SnF2-based fluoride ion electrolytes MSnF4 (M = Ba, Pb) for the application of room-temperature solid-state fluoride ion batteries

Journal Pre-proof SnF2-based fluoride ion electrolytes MSnF4 (M = Ba, Pb) for the application of roomtemperature solid-state fluoride ion batteries Lei Liu, Li Yang, Min Liu, Xiaolong Li, Dingsheng Shao, Kaili Luo, Xianyou Wang, Zhigao Luo PII:

S0925-8388(19)34229-X

DOI:

https://doi.org/10.1016/j.jallcom.2019.152983

Reference:

JALCOM 152983

To appear in:

Journal of Alloys and Compounds

Received Date: 3 July 2019 Revised Date:

2 November 2019

Accepted Date: 10 November 2019

Please cite this article as: L. Liu, L. Yang, M. Liu, X. Li, D. Shao, K. Luo, X. Wang, Z. Luo, SnF2based fluoride ion electrolytes MSnF4 (M = Ba, Pb) for the application of room-temperature solidstate fluoride ion batteries, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.152983. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

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SnF2-based fluoride ion electrolytes MSnF4 (M = Ba, Pb) for the

2

application of room-temperature solid-state fluoride ion batteries

3

Lei Liu, Li Yang, Min Liu, Xiaolong Li, Dingsheng Shao, Kaili Luo, Xianyou Wang∗,

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Zhigao Luo* National Base for International Science & Technology Cooperation, National Local Joint

5 6

Engineering Laboratory for Key Materials of New Energy Storage Battery, Hunan Province Key

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Laboratory of Electrochemical Energy Storage & Conversion, School of Chemistry, Xiangtan

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University, Hunan 411105, China

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Abstract:

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The fluoride ion batteries (FIBs) based on “fluoride ion shuttle” have received

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extensive attention due to their high energy density, safety and thermal stability.

13

Although the FIBs show above advantages, several challenges still remain to be

14

tackled, e.g., so far reported solid-state FIBs can only operate at the high temperature

15

like 150 °C and above. Herein, the SnF2-based fluoride ion conductors MSnF4 (M =

16

Ba, Pb) are prepared through high-energy ball-milling and annealing as solid

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electrolytes in solid-state FIBs. The morphology and structure of the solid electrolytes

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are studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM).

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Especially, the fluoride ion transporting characteristics of BaSnF4 and PbSnF4 solid

20

electrolytes are investigated by impedance spectroscopy (EIS). The results show that

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the high RT ionic conductivities of BaSnF4 (2.02×10−4 S cm−1) and PbSnF4 ∗

Corresponding author: Xianyou Wang, Tel: +86 731 58293377; Fax: +86 731 58292052.

E-mail address: [email protected] 1

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(5.44×10−4 S cm−1) make it possible to prepare the RT solid-state FIBs with Sn as an

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anode and BiF3 as a cathode. The initial discharging capacities of the Sn/PbSnF4/BiF3

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cell and the Sn/BaSnF4/BiF3 cell studied by galvanostatic charge/discharge cycling

25

are 175 mAh g−1 and 125 mAh g−1 at RT, respectively. The structural and chemical

26

characterization of the BiF3 composites and the Sn composites before and after

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discharge is examined in detail by X-ray diffraction (XRD) in order to explicate the

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charge/discharge behavior of the RT solid-state FIBs.

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Keywords: Fluoride ion battery; Solid electrolyte; Room-temperature fluoride ion

30

conductor; Activation energy; Fluoride ion transporting characteristics

31 32

1. Introduction

33

With the rapid development of new energy industry, many high energy density and

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high safety rechargeable battery systems have been put into research and development

35

as an important alternative to energy reserve [1-3]. Because of the high energy density

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and excessive security, rechargeable solid-state batteries are becoming key

37

technologies for electric vehicles, electronic products and electrical energy-storage

38

systems, etc [4-7]. The use of solid electrolytes plays an important role in improving

39

the battery power capacity, energy density, reliability and lifespan, especially safety

40

[8-12]. At present, a variety of the all solid-state batteries, including all solid-state

41

lithium ion battery, all solid-state sodium ion battery and all solid-state lithium

42

sulphur battery, are being studied and developed. Moreover, it has also been proved

43

that rechargeable solid-state fluoride ion batteries (FIBs) as a kind of new battery 2

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system can be designed according to an idea of a ‘fluoride shuttle’ [13, 14]. Fluoride

45

ions are predicted to be the best anionic conductors in FIBs since they are light,

46

univalent, a small ionic size and the highest electronegativity (3.98) of all elements.

47

However, such batteries are currently limited to operation at high temperatures

48

because suitable fluoride ion transporting solid electrolytes are known to have high

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ionic conductivity only at high temperatures [15-18]. Reddy and Fichtner firstly

50

confirmed that tysonite-La0.9Ba0.1F2.9 can be used as the solid electrolyte for the

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rechargeable solid-state FIBs, while the anode was composed of Ce and the cathode

52

was combined from various metal fluoride composite materials. Unfortunately, in all

53

cases, the batteries can only work at 150 °C or above [13, 19, 20]. The La0.9Ba0.1F2.9

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ionic conductivity (2.8×10−4 S cm-1 at 433 K) is not high enough at room temperature

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(RT) [20-22]. Recently, based on the liquid electrolyte with high fluoride ion

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conductivity and wide voltage window, V.K. Davis et al. demonstrated reversible

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electrochemical cycling of metal fluoride electrodes at room temperature whereby F–,

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not the metal cation, is the active ion shuttle [30]. Although further improvements are

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necessary, the development of liquid electrolytes opens new possibilities to operate a

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FIB at room temperature and increase the interest for developing new electrode

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materials as well. Thus, the solid-state FIBs, which used solid fluoride ion conductor

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as electrolyte and metal/metal fluoride pair as electrodes, must operate with the

63

increasing temperature.

64

In order to realize the RT operation of solid-state FIBs, it is critical to develop a

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novel type of fast fluoride ion conductors with high RT ionic conductivity. 3

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Fortunately, SnF2-based metal fluorides MSnF4 (M = Pb, Ba, Sr) fluoride ion

67

conductors including BaSnF4 and PbSnF4 exhibit a high RT ionic conductivity

68

[23-28]. Actually, substantial experiments have been conducted for the SnF2-based

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metal fluorides MSnF4 (M = Pb, Ba, Sr) fluoride ion conductors. Owing to the high

70

ionic conductivity at RT, these fluoride ion conductors have been used as solid

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electrolytes in various electrochemical devices and systems, especially in gas sensors,

72

fluorine pumps and gas phase electrolysers other than solid-state FIBs [23, 26]. For

73

instance, Reddy and Fichtner first demonstrated the use of BaSnF4 as the solid

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electrolyte for fluoride ion transport in rechargeable RT solid-state FIBs [29]. The

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tetragonal BaSnF4 solid electrolyte has a RT ionic conductivity of 3.5×10-4 S cm-1 that

76

can be employed to construct RT solid-state FIBs. They investigated the

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electrochemical properties of the Zn/BaSnF4/BiF3 and Sn/BaSnF4/BiF3 cells at

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temperatures of 25 °C, 60 °C, 100 °C, and 150 °C, and found that the initial discharge

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capacities can reach 120 mAh g−1 and 56 mAh g−1 at RT. The RT Sn/BaSnF4/BiF3

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cells can only work for 10 cycles at a current density of 10 µA cm−2. This work takes

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a pioneering step for the development of SnF2-based metal fluorides MSnF4 (M = Pb,

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Ba, Sr) fluoride ion conductors as solid electrolyte applied into rechargeable RT

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solid-state FIBs.

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The SnF2-based metal fluorides MSnF4 (M = Pb, Ba, Sr) with high ionic

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conductivity as fluoride ion conductors is decided by its special crystal structure [23,

86

30-32]. The fluoride ion conductor BaSnF4 is crystallized in a layered tetragonal

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structure in which an order of ···BaBaSnSnBaBa··· is parallel to the unit cell 4

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c-axis. The fluoride ion shuttle in BaSnF4 primarily occurs in the Ba-Sn and Sn-Sn

89

layers. Since the lone pair of electrons on SnF2 has a great influence on the fluoride

90

sublattice, BaSnF4 has a high fluoride ionic conductivity at RT. Among the Ba layers,

91

the fluoride ions are rigidly attached to the crystal lattice and exchange very slowly

92

with a group of mobile fluoride ions in the Ba-Sn and Sn-Sn layers, in which all

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fluoride ions participate during the conduction progression. The Ba-Ba layers, which

94

act as barriers to three-dimensional conductivity, lead to extremely anisotropic motion

95

in the layers. This motion is mainly by jumping into the sites of Ba-Sn rather than

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straightly occurring between that of the Ba-Ba sites. As a result, there is a lot of

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disorder in the fluoride ion sites. The combination of fluoride ions in the disordered

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fluorite-type sites and the interstitial sites may be the reason for the high ionic

99

conductivity. To sum up, the reason of SnF2-based metal fluorides MSnF4 (M = Pb,

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Ba, Sr) fluoride ion conductors with a much higher ionic conductivity is possibly due

101

to the disorders caused from the stereo-activity of the tin (II) lone pairs [23, 24].

102

Therefore, the fluoride ionic conductivity of the SnF2-based metal fluorides MSnF4

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(M = Pb, Ba, Sr) fluoride ion conductors is the highest at RT. In addition to Sn2+, it

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exists other cations (Sb3+, Bi3+, Pb2+, Tl+) that transfer the lone pair of electrons, with

105

comparable effects on ionic conductivity [27]. It provides a broad space for the

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application of fluoride ion conductors in rechargeable RT solid-state FIBs. The

107

structures are isomorphic for BaSnF4 and PbSnF4, and the latter one is an alleged

108

superionic conductor at RT, though the ionic conductivity of BaSnF4 shows one or

109

two orders of magnitude lesser than PbSnF4. Meanwhile, the abnormal high fluoride 5

110

ion mobility in the PbSnF4 polycrystalline samples has also been investigated via

111

complex impedance. PbSnF4 with the best performance in fluoride ion motion has

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been applied to design fast RT response amperometric oxygen sensor. For the

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structure of α-PbSnF4, fluoride atoms are axially stuck to the tin and located in the

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vacant sites, which can be employed to generate Frenkel flaws in fluorite structure.

115

Therefore, interstitials apply other blank sites, including the mid-plane in the two

116

layers, in which tin lone pairs are occupied [32-34]. Hence, the solid electrolyte

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PbSnF4 with the highest fluoride ionic conductivity in the family of SnF2-based solid

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electrolytes MSnF4 (M = Pb, Ba, Sr) is more conducive to the study of rechargeable

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RT solid-state FIBs.

120

Herein, we made an attempt to enable the superionic conductor BaSnF4 and PbSnF4

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as solid electrolytes in rechargeable RT solid-state FIBs systems. The high-energy

122

ball-milling is adopted to manufacture solid solutions of BaSnF4 and PbSnF4. X-ray

123

diffraction (XRD) and scanning electron microscopy (SEM) are employed to measure

124

the morphological, chemical and structural characterizations of the solid electrolytes.

125

In addition, electrochemical impedance spectroscopy (EIS) is applied to acquire the

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solid electrolyte ionic conductivity. The electrochemical cycling performance is also

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tested for solid-state FIBs based on BiF3 composite, Sn and BaSnF4 or PbSnF4 as

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cathode, anode and solid electrolyte, respectively. Finally, the structural and chemical

129

characterization of the BiF3 composite cathodes and Sn composite anodes before and

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after discharge are done in detail by X-ray diffraction (XRD) to demonstrate the

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charge-discharge behavior of the BiF3/Sn systems in RT solid-state FIBs. 6

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2. Experimental

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2.1 Preparation of electrolyte and electrode

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Preparation of electrolyte. It was dehydrated for 20h in the vacuum environment at

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80 °C for Bismuth trifluoride (BiF3, 98%), tin difluoride (SnF2, 99%), Barium

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fluoride (BaF2, 99%), Lead fluoride (PbF2, 99%), Sn powder (99%) and carbon

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nanotubes (CNTs) (95%) to eliminate the adsorbed moisture before ball milling. With

138

a zirconia vial in the argon atmosphere, the high-energy ball-milling was carried out

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from a planetary ball mill, with 12:1 of the powder ratio. Synthesis of precursor

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powders was according to the previous reports [31, 34]. Briefly, it was milled for 18 h

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at the speed of 500 rpm for the composites of BaF2 or PbF2 and SnF2. Afterward, the

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attained specimens were annealed at the temperature of 300 °C in an argon

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atmosphere, with the time of 2h.

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Preparation of electrode composites. Electrode composites and solid electrolytes

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were mixed by mechanical ball-milling in an argon environment and prepared with a

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zirconia vial via a planetary ball mill for 2 h at 200 rpm. The ratio of the ball to

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powder was 20:1. A composite cathode was synthesized based on the mixture of BiF3,

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CNTs, and BaSnF4 or PbSnF4 with a weight ratio of 40:10:50. The composite anode

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was prepared by mixing Sn, CNTs, and BaSnF4 or PbSnF4 together at a weight ratio

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of 50:10:40. In this work, the presented capacities were respective to the weight

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percentages for BiF3 in cathode. Generally, the average loading mass of the BiF3

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positive active material was about 1.0 mg cm-2.

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2.2 Material characterization 7

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In order to attain the phase composition of the materials, X-ray diffractometer

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(XRD, Model LabX-6000, Shimadzu, Japan) was scanned in the 2θ range of 10°–90°.

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The structure and morphology of the prepared specimens were measured via scanning

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electron microscopy (SEM, Hitachi S-4800).

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2.3 Electrochemical measurement and battery testing

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Impedance spectroscopy typically acquires the complex impedance to determine

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ionic conductivity of a compound with a blocking electrode configuration. A solid

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electrolyte can be placed among the two equivalent electrodes without ionic

162

conductivity [35-37]. Ionic conductivity of solid electrolytes was evaluated by EIS

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(CHI660E electrochemical workstation) on a 14 mm diameter pellet, which was

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constrained from a 3 tons force for 2 min and coated with an Au layer (thickness, 20

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nm) on both sides for ion-blocking electrodes. The electrochemical impedance

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spectroscopy (EIS) measurement temperature range was from 20 ℃ to 80 ℃ from

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frequeny of 1×106 to 1×10−2 Hz with a perturbation of 10 mV. The ionic conductivity

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(σDC) of solid electrolyte was calculated according to the following equation: = ×

169 170 171

(1)

where l is the solid electrolyte thickness, R is the bulk electrolyte resistance, and A is the area of electrolyte.

172

The full cell was constructed under an argon protective environment by

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compressing the powders of the cathode, electrolyte, and the anode. The cathode was

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forced to the top and the electrolyte was sandwiched. Sufficient materials were put to

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circumvent possible short circuit. The main preparation process of RT solid-state FIBs 8

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and the testing devices were shown in Fig. 1. The cell was circulated at RT, with a

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current density of 12.7 µA cm−2 and a voltage window of 0.01−1.5 V.

178 179

Fig. 1. Preparation schematic and photos of electrolytes, electrode materials and solid-state FIB.

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3. Results and Discussion

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3.1 Structure and ionic conductivity of MSnF4 (M= Ba, Pb)

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The XRD patterns of BaSnF4 and PbSnF4 specimens after mechanical milling and

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annealing are depicted in Fig. 2. As shown in Fig. 2a, it can be found that BaF2

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completely reacts with SnF2 to obtain a new cubic fluorite-type phase for BaSnF4 (Fig.

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2a, lower layer), and the cubic structure of BaSnF4 has a preferential orientation along

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the (102) direction, which confirms that the as-prepared samples have polycrystalline

187

nature without additional reflections. Besides, the as-prepared sample shows the

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reflections at 2θ angles of 24.84°, 28.88°, 41.44° and 48.91°, which are corresponding

189

to the planes of (102), (110), (200) and (212). Briefly, the characteristic peaks of

190

BaSnF4 are crystallized in a cubic structure, indicating that Ba2+ and Sn2+ ions are

191

mutually disordered. After the prepared materials was annealed at 300 ℃ for 2 h in Ar

192

protective atmosphere, the cubic structures alters to a tetragonal-type for BaSnF4 (Fig. 9

193

2a, top layer). Accordingly, the annealing treatment leads to non-negligible narrowing

194

of the XRD peaks due to the grain growth. These results demonstrate that the cubic

195

disorder is a metastable phase and is converted to a tetragonal one via annealing the

196

specimen at 300 ℃. These results are consistent with the reported data [23, 24, 30].

197

The similar results are also attained in the PbF2-SnF2 system in Fig. 2b, where

198

tetragonal phase is acquired after milling of PbF2 and SnF2 (α-PbSnF4, Fig. 2b, lower

199

layer). The X-ray single crystal diffraction and neutron Rietveld analysis indicated

200

that the structure of α-PbSnF4 was substantially the same as the structure of literatures

201

[34, 38]. After annealing at 300 ℃, the phase is transformed into the orthorhombic

202

PbSnF4 (o-PbSnF4, Fig. 2b, top layer). The diffraction pattern of o-PbSnF4 has very

203

high similarity to the α-phase pattern structure. The chief difference exists in the

204

certain peak splitting, which means the symmetric reduction. The alterations can be

205

completely elucidated in the orthorhombic unit-cell. As suggested by Perez et al., no

206

further symmetry break needs to be added to the monoclinic system [38]. It should be

207

emphasized that it is a small structural transition from tetragonal to orthorhombic

208

(only one break for symmetry).

209 10

210

Fig. 2. XRD patterns of (a) BaSnF4 and (b) PbSnF4 after ball-milled and annealed treatment.

211

It has been predicted that excessive mechanical grinding applied in the preparation

212

process will lead to great stresses, high disordering degree, and insignificant

213

crystallinity of the material [30]. Usually, the specimen annealing at high temperature

214

can not only increase the number of defects, but also promotes the grain growth,

215

hence improving the ionic conductivity. Fig. 3 shows the surface morphology of

216

BaSnF4 and PbSnF4 samples after mechanical milling and annealing. The SEM

217

images of the ball-milled BaSnF4 (Fig. 3a) and PbSnF4 (Fig. 3c) samples show the

218

fine particle agglomeration, which is possibly due to the excessive milling during the

219

material preparation, resulting in large stress and disordering degree of materials. On

220

annealing process, the grains grow into bigger sizes with a widespread range, which

221

can improve the bonding from the adjacent particles and leads to the strength

222

improvement, as shown in Fig. 3b and Fig. 3d. Consequently, the annealing is the

223

most commonly used method to enhance the ionic conductivity of solid solutions.

11

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Fig. 3. The SEM pictures of (a) BaSnF4 after ball-milled, (b) BaSnF4 after annealed treatment,

226

and (c) PbSnF4 after ball-milled, (d) PbSnF4 after annealed treatment.

227

In order to investigate the thickness and morphology of the solid electrolyte pellets,

228

cross-section SEM analysis is carried out. It can be clearly observed from the SEM

229

image in Fig. 4a that the thickness of the solid electrolyte is around 1540 µm. The

230

density of BaSnF4 (Fig. 4b) and PbSnF4 (Fig. 4c) solid electrolytes are further

231

increased with the increase of the pressure. It is well known that the increase of

232

density contributes to continue increasing the ionic conductivity of solid electrolyte

233

pellets. Fig. 4d indicates Nyquist plots for BaSnF4 solid electrolyte and PbSnF4 solid

234

electrolyte, as well as the corresponding equivalent circuit (the inset of Fig. 4d). The

235

EIS spectra are recorded by a concave semicircle and an oblique straight line at high

236

and low frequencies, respectively. In general, the impedance spectrum of 12

237

polycrystalline electrolyte materials should be two semicircular arcs, the first one

238

belonging to the grain and the second one to the grain boundary (low frequencies)

239

[39]. However, only a concave (non-ideal) semicircle can be seen in Fig. 4d, and the

240

centre of the circle is below the real axis. This non-ideal behavior may be due to: (i)

241

the existence of relaxation times distribution in the response of bulk (ii) a distortion

242

from other relaxations, e.g. grain boundary relaxation. However, the experimental

243

results in this work imply that the concave semicircle in the impedance spectra agrees

244

to the bulk behavior of the samples. As described below, the impedance spectra are

245

not contributed to from the boundary without grain. Total resistance is established

246

from the Nyquist plot that can be fitted via an equivalent circuit as illustrated in Fig.

247

4d. The equivalent circuit is composed of a total resistance (R1) and a constant phase

248

element (CPE1) that are parallel combined and generated by the boundaries of bulk

249

and grain, subsequently combining with a constant phase element (CPE2) from

250

blocking electrodes. The fluoride ionic conductivity of BaSnF4 solid electrolyte is

251

2.02×10−4 S cm−1 from the calculation and PbSnF4 solid electrolyte is 5.44×10−4 S

252

cm−1 at room temperature.

253 254 255

A hopping mechanism describing point defects can be used to explain DC ionic conductivity of majorities of ionic conductors, correspondingly: =

(2)

256

a, q, k, T and Nc are corresponding to the hopping distance, the electronic charge,

257

the Boltzmann constant, the absolute temperature, and the charge carrier

258

concentration. Nc and fh are following: 13

259

=

260

"# = "

−∆ −∆

⁄

!

(3)

$⁄

!

(4)

261

N0, ∆H0, f0 and ∆Hm represent to the infinite temperature carrier concentration, the

262

mobile charge carrier creation enthalpy, the fundamental vibrational frequency, and

263

the migration enthalpy, respectively.

264 265

Based on the above equations, the following equation can be derived: =

% %

&−

∆'% (∆')

*=

+%

&−

,-

*

(5)

266

σ0 gives the pre-exponential factor and Ea refers to the activation energy.

267

The investigation of DC ionic conductivity can generally be plotted based on the

268

diagram of log (σDC·T) versus 1/T, which can consequently acquire the activation

269

energy Ea (slope) and the pre-exponential factor σ0 (intercept). The carrier

270

concentration is typically independent with temperature (ΔH0→0), especially for

271

doped ionic conductors in the extrinsic region due to the doping defect number

272

significantly surpasses the intrinsic defect number. Meanwhile, the migration enthalpy

273

is close to the activation energy Ea. Ea for BaSnF4 solid electrolyte and PbSnF4 solid

274

electrolyte established from Arrhenius plot fitted via a straight line are found to be

275

0.23 eV and 0.26 eV as shown in Fig. 4e, which is similar to the reported value [23,

276

26]. Therefore, the low activation energy of BaSnF4 and PbSnF4 can reciprocate the

277

high ion conductivity of the solid electrolyte and provide the good condition for the

278

preparation of RT solid-state FIBs.

14

279 280

Fig. 4. The SEM images of cross-sectional views of (a) solid electrolyte, (b) BaSnF4, (c) PbSnF4,

281

(d) the impedance spectra of BaSnF4 and PbSnF4 solid electrolytes obtained at RT, (e) Arrhenius

282

plots for the ionic conductivity of BaSnF4 and PbSnF4 solid electrolytes. The impedance

283

measurements are conducted to acquire the signs of ionic conductivities measured from, with the

284

straight lines for fitting to calculate activation energies.

285

3.2 Characterization of electrode composites

286

Fig. 5 is the XRD results of the electrode composite materials. BiF3 is selected as

287

cathode material because of its high ionic conductivity for fluoride ion at RT and its

288

noble reversibility in solid-state FIBs. Besides, Sn is selected as anode materials

289

because it predicts electrochemical stability against BaSnF4 solid electrolytes and

290

PbSnF4 solid electrolytes. In addition, SnF2 is a Sn discharge product of solid-state

291

FIBs and is a typical notable fluoride ion conductor. Fig. 5a and Fig. 5b show the

292

XRD patterns of the as-prepared BiF3/BaSnF4 or PbSnF4/CNTs cathode composite

293

and Sn/BaSnF4 or PbSnF4/CNTs anode composite. After the cathode preparation by 15

294

ball milling, two phases of the BaSnF4 or PbSnF4 solid electrolyte and the active

295

material BiF3 (PDF No. 15-0053) are observed. No impurity diffraction peak is

296

detected, indicating that BiF3 does not react with the electrolyte material during the

297

ball-milling process. Sn anode composites show the same situation. As shown in the

298

XRD patterns of Sn/BaSnF4 or PbSnF4/CNTs anode composites in Fig. 5a and Fig. 5b,

299

BaSnF4 or PbSnF4 solid electrolyte powders are found in all the composites except

300

active materials. Other XRD peaks are not found for the Sn anode electrode

301

composites, indicating that intermediate crystalline phases are not formed during

302

milling process of the active materials and the solid electrolyte.

303 304

Fig. 5. The XRD curves of the electrode composites: (a) BaSnF4, Sn composite anode, and BiF3

305

composite cathode, (b) PbSnF4, Sn composite anode, and BiF3 composite cathode.

306

Fig. 6 is the SEM images of BiF3/BaSnF4/CNTs, Sn/BaSnF4/CNTs, BiF3/PbSnF4/CNTs

307

composite and Sn/PbSnF4/CNTs. It can be seen the homogeneous distribution of active

308

material, solid electrolyte and CNTs. Solid electrolyte powders are added to increase

309

the effective transport of fluoride ions in the electrode materials. As shown in Fig. 6,

310

the CNTs show a network structure. CNTs with network structure can not only

311

enhance the electronic conductivity of electrode materials, but also provide buffer 16

312

space for the volume change of active materials in the electrode reaction process.

313 314 315

Fig. 6. The SEM images of (a) BiF3/BaSnF4/CNTs composite, (b) Sn/BaSnF4/CNTs composite,

316

(c) BiF3/PbSnF4/CNTs composite and (d) Sn/PbSnF4/CNTs composite.

317

3.3 Performance of the RT solid-state FIBs

318

The galvanostatic discharge/charge profiles, cyclic performance and coulombic

319

efficiency for the Sn/BaSnF4/BiF3 cells and Sn/PbSnF4/BiF3 cells for the first ten

320

cycles at RT are exhibited in Fig. 7. Here, the presented capacities are respective to

321

the weight percentages for BiF3 in cathode. In Fig. 7a, the Sn/BaSnF4/BiF3 cell

322

delivers the initial discharge capacity of 125 mAh g−1, and this value is 41% of the

323

theoretical specific capacity (302 mAh g−1), indicating incomplete transformation of

324

BiF3 to Bi. On even ground, the Sn/PbSnF4/BiF3 cell delivers the first discharge 17

325

capacity of 175 mAh g−1 (in Fig. 7c), and that is 58% of the theoretical value (302

326

mAh g−1), suggesting more reactions of BiF3 to Bi. The capacities of Sn/PbSnF4/BiF3

327

cells obtained for the first cycle are higher than that of Sn/BaSnF4/BiF3 cells of 50

328

mAh g−1. It may be caused by the higher conductivity measured for PbSnF4 compared

329

to BaSnF4. However, the theoretical capacities of the Sn/BaSnF4/BiF3 cells and the

330

Sn/PbSnF4/BiF3 cells have not been reached, which may be due to the low fluoride

331

ion motion and the difficulty in transferring charge from the solid electrolyte to the

332

electrodes. At the same time, it should be noticed that the theoretical capacity of the

333

composite electrodes is not fully utilized, which may be caused by the absence of

334

mass and/or electron transfer interactions between the isolated grains of active

335

material. This phenomenon will be further verified in the next section.

336

On subsequent cycling the capacity of Sn/PbSnF4/BiF3 cells fades progressively

337

and is only 80 mAh g-1 after 10 cycles as shown in Fig. 7c, and the Sn/BaSnF4/BiF3

338

cells is 53 mAh g-1 in Fig. 7a. The FIBs using Sn/BaSnF4/BiF3 system prepared by

339

Reddy and Fichtner exhibits a high first discharge capacity of 120 mAh g−1, however

340

with rapid fading upon cycling [29]. Generally speaking, the capacity gradually fades

341

in the two solid-state FIB cells. It is a great challenge for solving the fading issue of

342

capacity in the cycling process in solid-state FIBs. In an entirely solid-state

343

environment, the capacity fading may be because of the diminishing of

344

electrode/electrolyte interfacial contact or the deterioration of interparticle contacts

345

caused by the variation in the volume of the BiF3 and Sn electrodes during the process

346

of discharge and charge [14]. In other words, the high coulombic efficiency of the two 18

347

battery systems can illustrate this problem from another perspective. As seen in Fig.

348

7b and Fig. 7d, the coulombic efficiency of two battery systems reaches above 90%,

349

indicating

350

discharging/charging process. It can be possibly explained as follows: the electrode

351

materials can suffer reversible volume alterations in the discharge and charge process,

352

thus weakening the contact between the electrode and the electrolyte. The loss of

353

contact between the particles and the capacity fade is a notorious fact of the

354

conversion materials. At the same time, compared with the BaSnF4 system, the

355

PbSnF4 system presents higher discharge capacity but the worse cycle stability. The

356

possible reason is that the activation energy Ea of PbSnF4 (0.26 eV) is slightly higher

357

than that of BaSnF4 (0.23 eV). The Fig. 7e shows each of the DF- values of

358

Sn/BaSnF4/BiF3 cells and Sn/PbSnF4/BiF3 cells. The GITT results further prove that

359

the Sn/PbSnF4/BiF3 cells has better ion diffusion property than Sn/BaSnF4/BiF3 cells.

360

The GITT results not only explain the reason why the electrochemical performance of

361

Sn/PbSnF4/BiF3 cells is better than that of Sn/BaSnF4/BiF3 cells, but also explain the

362

reason why the cycling performance of solid-state FIBs is poor. Further, the fourth

363

cycle CV curves of Sn/PbSnF4/BiF3 cells and Sn/BaSnF4/BiF3 cells in Fig 7f exhibit

364

the influence of different electrolytes for the voltage and polarization of the cells. The

365

Sn/PbSnF4/BiF3 cells shows a high peak current, and the peak potential of

366

Sn/PbSnF4/BiF3 cells shifts slightly to the left, indicating that the polarization of

367

Sn/PbSnF4/BiF3 cells is smaller than that of Sn/BaSnF4/BiF3 cells. Although the

368

capacity fades rapidly in cycling process, the operation of rechargeable RT solid-state

almost

complete

reversible

reaction

19

of

Bi

to

BiF3

in

one

369

FIBs using BaSnF4 or PbSnF4 solid electrolytes could be demonstrated.

370 371

Fig. 7. Electrochemical studies are attained at 25 °C for a current density of 12.7 µA cm−2 on

372

Sn/BaSnF4/BiF3 cells, (a) the curves for the first ten galvanostatic discharge−charge, (b) cycling

373

behavior and coulombic efficiency, and Sn/PbSnF4/BiF3 cells, (c) the curves for first ten

374

galvanostatic discharge−charge, (d) cycling behavior and coulombic efficiency, (e) the calculated

375

DF- values for the BiF3/MSnF4/Sn (M=Ba, Pb) cells, (f) the fourth cycle CV curves of

376

Sn/PbSnF4/BiF3 cells and Sn/BaSnF4/BiF3 cells. 20

377

3.4 XRD analysis of before and after discharge electrode composites

378

In order to gain insight into the conversion reaction mechanism of the RT

379

solid-state FIBs during the discharge and charge process, XRD measurements are

380

conducted on pristine, discharged, and charged BiF3 and Sn electrode composites in

381

the first electrochemical cycle process. Fig. 8a and Fig. 8c indicate the XRD pattern

382

of BiF3 composites before and after the process of discharge. In Fig. 8a and Fig. 8c,

383

the diffraction peaks are observed for BiF3 and BaSnF4 or PbSnF4. When discharging

384

to 0.01 V, the diffraction peaks of BiF3 will disappear incompletely, indicating

385

incomplete conversion at the cathode. Because of the occurrence of isolated grains of

386

active material not appropriately coupled by mass and/or electron transfer, the

387

incomplete reaction happened at cathode causes low initial capacity, which is

388

confirmed in the above section. New peaks appeared in the XRD pattern can be

389

marked as Bi metal (PDF No. 44-1246), confirming the BiF3 to Bi reduction when the

390

discharge is finished. Both discharged electrode composites exhibit the electrolyte

391

sturdy pattern and the Bi metal reflections. Besides, Bi diffraction peaks are strong

392

and sharp in BiF3 composites. The Bi diffraction peak sharpness can be possibly

393

caused by the particle crystallization. The presence of Bi elements of BiF3 solid

394

solution electrodes indicates that Bi metal particles are precipitated in the lattice. Fig.

395

8b and Fig. 8d are the XRD curves of Sn electrode composites before and after the

396

discharge. Before this discharge process, XRD patterns show the diffraction peaks of

397

Sn and solid solution electrolytes, respectively. After discharge, the discharged Sn

398

electrode composites show the appearance of SnF2 diffraction peaks (PDF No. 21

399

15-0744). Because of the excess amount of anode material adopted, the diffracting

400

peaks for Sn still can be found after the discharge. This situation is consistent with the

401

BiF3 composites.

402 403

Fig. 8. The XRD patterns of BaSnF4 (a) BiF3 composite, (b) Sn composite, and PbSnF4 (c) BiF3

404

composite, (d) Sn composite before and after the discharge.

405

4. Conclusions

406

The ionic conductivities for BaSnF4 and PbSnF4 solid solutions have been

407

investigated for polycrystalline specimens manufactured via high-energy ball-milling.

408

In addition, rechargeable RT solid-state FIBs are also confirmed in the form of a cell

409

composed of a Sn electrode associated to BiF3 couple as a negative electrode through

410

the BaSnF4 or PbSnF4 solid electrolytes. The ionic conductivity of PbSnF4 solid

411

electrolyte reaches 5.44 × 10−4 S cm−1 at RT, while BaSnF4 solid electrolyte reaches 22

412

2.02 × 10−4 S cm−1. The activation energies (Ea) for BaSnF4 solid electrolyte and

413

PbSnF4 solid electrolyte are found to be 0.23 eV and 0.26 eV. BaSnF4 and PbSnF4

414

solid electrolytes show promising performance on fluoride ion transferring at RT,

415

which are capable of constructing an RT solid-state FIBs. Two novel solid-state FIB

416

systems with BaSnF4 or PbSnF4 solid electrolyte have been discussed. At RT, Sn/BiF3

417

systems exhibit first discharge capacity of 175 mAh g−1 and 125 mAh g−1. The RT

418

solid-state FIBs deliver small average voltage because of the minor voltage redox

419

couples (Sn/BiF3) and the capacity decayed rapidly. In the future works, it should

420

make efforts to find the combinations of appropriate anode and cathode, thus possibly

421

offering high specific energy, high voltage and long durability.

422 423

Acknowledgments

424

The work was supported by the National Natural Science Foundation of China (No.

425

51272221), the Key Project of Strategic New Industry of Hunan Province (grant

426

numbers 2016GK4005, 2016GK4030), Xiangtan University Innovation Foundation

427

for Postgraduate (No. CX2018B057), Hunan province Innovation Foundation for

428

Postgraduate (No. CX2018B372) and Hunan Provincial Natural Scientific Foundation

429

of China (No. 2019JJ50615).

430

References

431

[1] J.M. Tarascon, M. Armand, Nature 414 (2001) 359.

432

[2] J.B. Goodenough, K.-S. Park, J. Am. Chem. Soc. 135 (2013) 1167-1176.

433

[3] K. Zhang, X. Han, Z. Hu, X. Zhang, Z. Tao, J. Chen, Chem. Soc. Rev. 44 (2015) 23

434

699-728.

435

[4] S.-H. Chung, A. Manthiram, Joule 2 (2018) 710-724.

436

[5] R. Mohtadi, M. Matsui, T.S. Arthur, S.-J. Hwang, Angew. Chem. Int. Ed. 51

437

(2012) 9780-9783.

438

[6] J. Cabana, L. Monconduit, D. Larcher, M.R. Palacín, Adv. Mater. 22 (2010)

439

E170-E192.

440

[7] V.K. Davis, C.M. Bates, K. Omichi, B.M. Savoie, N. Momčilović, Q. Xu, W.J.

441

Wolf, M.A. Webb, K.J. Billings, N.H. Chou, S. Alayoglu, R.K. McKenney, I.M.

442

Darolles, N.G. Nair, A. Hightower, D. Rosenberg, M. Ahmed, C.J. Brooks, T.F.

443

Miller, R.H. Grubbs, S.C. Jones, Science 362 (2018) 1144-1148.

444

[8] D.W. McOwen, S. Xu, Y. Gong, Y. Wen, G.L. Godbey, J.E. Gritton, T.R. Hamann,

445

J. Dai, G.T. Hitz, L. Hu, E.D. Wachsman, Adv. Mater. 30 (2018) 1707132.

446

[9] A. Manthiram, X. Yu, S. Wang, Nat. Rev. Mater. 2 (2017) 16103.

447

[10] J.-J. Kim, K. Yoon, I. Park, K. Kang, Small Methods 1 (2017) 1700219.

448

[11] Q. Zhang, K. Liu, F. Ding, X.J.N.R. Liu, Nano Res. 10 (2017) 4139-4174.

449

[12] Y. Tian, T. Shi, W.D. Richards, J. Li, J.C. Kim, S.-H. Bo, G. Ceder, Energy

450

Environ. Sci. 10 (2017) 1150-1166.

451

[13] M. Anji Reddy, M. Fichtner, J. Mater. Chem. 21 (2011) 17059-17062.

452

[14] D.T. Thieu, M.H. Fawey, H. Bhatia, T. Diemant, V.S.K. Chakravadhanula, R.J.

453

Behm, C. Kübel, M. Fichtner, Adv. Funct. Mater. 27 (2017) 1701051.

454

[15] J. Chable, B. Dieudonné, M. Body, C. Legein, M.-P. Crosnier-Lopez, C. Galven,

455

F. Mauvy, E. Durand, S. Fourcade, D. Sheptyakov, M. Leblanc, V. Maisonneuve, A. 24

456

Demourgues, Dalton Trans. 44 (2015) 19625-19635.

457

[16] J. Chable, A.G. Martin, A. Bourdin, M. Body, C. Legein, A. Jouanneaux, M.P.

458

Crosnier-Lopez, C. Galven, B. Dieudonné, M. Leblanc, A. Demourgues, V.

459

Maisonneuve, J. Alloys and Compd. 692 (2017) 980-988.

460

[17] C. Rongeat, M. Anji Reddy, T. Diemant, R.J. Behm, M. Fichtner, J. Mater. Chem.

461

A 2 (2014) 20861-20872.

462

[18] B. Dieudonné, J. Chable, M. Body, C. Legein, E. Durand, F. Mauvy, S. Fourcade,

463

M. Leblanc, V. Maisonneuve, A. Demourgues, Dalton Trans. 46 (2017) 3761-3769.

464

[19] L. Zhang, M. Anji Reddy, P. Gao, M. Fichtner, J. Alloys and Compd. 684 (2016)

465

733-738.

466

[20] L. Zhang, M.A. Reddy, M.J.J.o.S.S.E. Fichtner, J. Solid State Electrochem. 22

467

(2018) 997-1006.

468

[21] A. Roos, D.R. Franceschetti, J. Schoonman, J. Phys. Chem. Solids 46 (1985)

469

645-653.

470

[22] C. Rongeat, M. Anji Reddy, R. Witter, M. Fichtner, ACS Appl. Mater. Interfaces

471

6 (2014) 2103-2110.

472

[23] F. Preishuber-Pflügl, V. Epp, S. Nakhal, M. Lerch, M. Wilkening, physica status

473

solidi (c) 12 (2015) 10-14.

474

[24] L.N. Patro, N. Ravi Chandra Raju, S.R. Meher, K.J.A.P.A. Kamala Bharathi,

475

Appl. Phys. A 112 (2013) 727-732.

476

[25] S. Chaudhuri, F. Wang, C.P. Grey, J. Am. Chem. Soc. 124 (2002) 11746-11757.

477

[26] E. Murray, D.F. Brougham, J. Stankovic, I. Abrahams, J. Phys. Chem. C 112 25

478

(2008) 5672-5678.

479

[27] N.I.J.I.M. Sorokin, Inorg. Mater. 40 (2004) 989-997.

480

[28] G. Dénès, Mater. Res. Bull. 15 (1980) 807-819.

481

[29] I. Mohammad, R. Witter, M. Fichtner, M. Anji Reddy, ACS Appl. Energy Mater.

482

1 (2018) 4766-4775.

483

[30] M.M. Ahmad, Y. Yamane, K. Yamada, J. Appl. Phys. 106 (2009) 074106.

484

[31] G. Dénès, J. Hantash, A. Muntasar, P. Oldfield, A.J.H.I. Bartlett, Hyperfine

485

interact. 170 (2006) 145-158.

486

[32] R. Kanno, S. Nakamura, Y. Kawamoto, Solid State Ionics 51 (1992) 53-59.

487

[33] A.B. Podgorbunsky, S.L. Sinebryukhov, S.V. Gnedenkov, Physics Procedia 23

488

(2012) 94-97.

489

[34] G. Dénès, G. Milova, M.C. Madamba, M. Perfiliev, Solid State Ionics 86-88

490

(1996) 77-82.

491

[35] A.K. Ivanov-Shits, N.I. Sorokin, P.P. Fedorov, B.P. Sobolev, Solid State Ionics 31

492

(1989) 269-280.

493

[36] J.T.S. Irvine, D.C. Sinclair, A.R. West, Adv. Mater. 2 (1990) 132-138.

494

[37] G.A. Ragoisha, N.P. Osipovich, A.S. Bondarenko, J. Zhang, S. Kocha,

495

A.J.J.o.S.S.E. Iiyama, J. Solid State Electrochem. 14 (2010) 531-542.

496

[38] G. Pérez, S. Vilminot, W. Granier, L. Cot, C. Lucat, J.-M. Réau, J. Portier, P.

497

Hagenmuller, Mater. Res. Bull. 15 (1980) 587-593.

498

[39] D.P. Almond, A.R. West, Solid State Ionics 9-10 (1983) 277-282.

499

[40] E.F. Hairetdinov, N.F. Uvarov, J.M. Reau, P. Hagenmuller, Physica B 244 (1998) 26

500

201-206.

501

27

Highlights: BaSnF4 and PbSnF4 solid electrolytes are manufactured via high-energy ball-milling. The ionic conductivity of BaSnF4 solid electrolyte is 2.02×10−4 S cm−1 and PbSnF4 solid electrolyte is 5.44×10−4 S cm−1 at RT. The activation energies for BaSnF4 and PbSnF4 solid electrolytes are 0.23 eV and 0.26 eV. Two novel solid-state FIB systems with BaSnF4 or PbSnF4 solid electrolyte are discussed. At RT, Sn/BiF3 systems exhibit first discharge capacity of 175 mAh g−1 and 125 mAh g−1.

The authors declare no competing financial interests.