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Reduction Depth Dependent Structural Reversibility of Sn3(PO4)2 Lijuan Fan, Xianwei Guo, Lian Shen, Gaojing Yang, Shuai Liu, Na Tian, Zhaoxiang Wang, and Liquan Chen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00024 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017

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ACS Applied Energy Materials

Reduction Depth Dependent Structural Reversibility of Sn3(PO4)2 Lijuan Fan1,2, Xianwei Guo3, Lian Shen1,2,4, Gaojing Yang1,2, Shuai Liu1,2, Na Tian1,2,5, Zhaoxiang Wang*,1,2, Liquan Chen1,2 1. Key Laboratory for Renewable Energy, Chinese Academy of Sciences; Beijing Key Laboratory for New Energy Materials and Devices; Beijing National Laboratory for Condensed Matter Physics; Institute of Physics, Chinese Academy of Sciences, P. O. Box 603, Beijing 100190, China 2. School of Physical Sciences, University of Chinese Academy of Sciences, P. O. Box 603, Beijing 100190, China 3. College of Materials Sciences and Engineering, Beijing University of Technology, Beijing 100124, China 4. Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes; National Laboratory of Mineral Materials; School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China 5. Pulead Technology Industry Co. Ltd, Beijing 102200, China *Corresponding Author: Tel: +86-10-82649050; E-mail: [email protected]

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KEYWORDS: lithium storage, Sn3(PO4)2, cut-off potential, reversibility, transportation

ABSTRACT: Conversion reaction is an important way of lithium storage. Tin-based compounds have been regarded as promising anode materials for lithium ion batteries due to their high specific capacities. Herein, we report the structural reversibility of Sn3(PO4)2 after conversion and Li-Sn alloying reactions. It is found that the reversibility of Sn3(PO4)2 is highly dependent on the cut-off discharge potential. The conversion reaction of Sn3(PO4)2 is partially reversible when it is discharged to 1.55 and then recharged to 3.00 V but is not between 0.00 and 3.00 V.

INTRODUCTION Intercalation and alloying are two of the most important forms of lithium storage and have been studied for decades.1-5 Typical intercalation materials include LiCoO2 and graphite while silicon (Si) stores lithium by alloying/de-alloying reactions. Intercalation-based materials exhibit great rate and cycling performances as these materials uptake and release Li-ions without renucleation. However, their capacity is low due to limited space in which the guest ions can be stored topotactically. In contrast, the specific capacity of the alloying materials is much higher. Nevertheless, the alloying materials suffer from large volume changes during lithiation and delithiation. As a result, these materials usually exhibit a short cycle life. A common feature of both the intercalation and alloying materials is that their structural stability is highly dependent on the cut-off potential. For example, irreversible phase transition occurs as the layered LiCoO2 is charged over 4.50 V (vs. Li+/Li) and stacking faults appear in the cycled LiCoO2 particles


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above 4.70 V,6 resulting in fast capacity fading. Similarly, the cycling performance of the Si electrode can be improved by limiting the cut-off discharge potential or controlling the storage capacity.7-10 Conversion is another important way of lithium storage and is rather new in comparison with the above two. It attracts much attention with high specific capacity as it often involves transference of more than one electron.11-12 However, a number of challenges remain to be solved for the conversion-based electrode materials, including potential hysteresis, low initial coulombic efficiency, and structural irreversibility. Most of the conversion reactions are poorly reversible unless the material is coated with carbon or contains transition-metals as the catalyst.13-14 For example, the electrochemical reduction of SnO2 was believed irreversible (with metallic tin and Li2O as the reduction products).15 However, partial oxidation of nanoscaled metallic tin to SnO was realized after SnO2 was coated with carbon.13 Complete oxidation of the metallic tin is available (tin is electrochemically oxidized to SnO2) in Ni2SnO4, with the reduced nanosized

metallic

nickel

as

the

catalyst

(Ni2 SnO4 +8Li+ +8e- → 2Ni+Sn+4Li2 O ↔ 2NiO+SnO2 +8Li+ +8e- ).13-14 Other factors that affect the conversion reaction pathways include surface chemistry, particle morphology as well as structure of the starting material.16-17 Cut-off potential is another factor that influences the reversibility of the conversion,18-19 but few authors focus their studies on the cut-off discharge potential.20-21 Herein, we reported the impacts of cut-off discharge potential on the structural reversibility of Sn3(PO4)2, an anode material that stores Li-ions by both conversion and Li-Sn alloying reactions.22 Characterization of structural evolution reveals that the conversion reaction of


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Sn3(PO4)2 is partially reversible when it is cycled between 1.55 and 3.00 V but becomes irreversible if the material is discharged to 0.00 V. EXPERIMENTAL SECTION Material preparation. Sn3(PO4)2 was prepared by a simple precipitation method. In a typical synthesis, SnCl2 was dissolved in deionized water followed by addition of diluted HCl to buffer the hydrolysis of SnCl2. Stoichiometric Na2HPO4 was dissolved in deionized water and slowly added into above SnCl2 solution with a Sn2+/(PO4)3+ molar ratio of 3:2. The obtained suspension was magnetically stirred for 3 h at room temperature. Then the precipitate was separated by filtration, washed with deionized water and dried at 80 °C. Thus resulting white powder was calcinated at 550 °C for 10 h under Ar atmosphere to obtain Sn3(PO4)2. Electrochemical evaluation. N-methyl-pyrrolidone slurry containing 65 wt% Sn3(PO4)2, 20 wt% super P (SP) and 15 wt% polyvinylidene fluoride (PVDF) was bladed onto a Cu foil as the working electrode. Then the resultant electrode was dried at 120 °C in a vacuum oven for overnight. Test cells were assembled in an Ar-filled glove box with lithium foil as the counter electrode, 1.0 mol L-1 LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 v/v) as electrolyte, and Celgard 2400 as separator. The cells were galvanostatically cycled within different potential ranges (vs. Li+/Li) at a current density of 40 mAh g-1 (all the current density and capacities in this work were based on the mass of Sn3(PO4)2 unless otherwise specified) on a Land BT2000 battery tester (Wuhan, China) at room temperature. The cyclic voltammetry (CV) was recorded on a CHI600D electrochemical workstation at a scanning rate of 0.05 mV s-1. The cycled cells were disassembled in the glove-box, and the working electrode was rinsed with DMC repeatedly and maintained under an airtight seal for other tests.


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Physical characterization. The morphology of the pristine Sn3(PO4)2 was characterized on a Hitachi S-4800 field-emission scanning electron microscope (SEM). X-ray diffraction (XRD) was collected on a Bruker D8 Advance X-ray diffractometer (λCu Kα =1.54056 Å) to recognize the charge/discharge products. The structure of the charge/discharge products was investigated using high-resolution transmission electron microscope (HRTEM; Tecnai G2 F20 U-TWIN at a 200-kV acceleration voltage) and Fourier-transform infrared spectrometer (FTIR; VERTEX 70 V, Bruker, Germany). X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific ESCALAB 250Xi equipped with a monochromatized Al Kα source to examine the oxidation states of tin. RESULTS AND DISCUSSION The XRD pattern of as-prepared sample (Figure 1a) is indexed to Sn3(PO4)2 (JCPDS No.700391). However, the relative intensity of some peaks, especially those between 20° and 30°, differs from that of the reference, indicating the preferential crystal orient growth of Sn3(PO4)2. The SEM image (Figure S1) shows that the particle size of the as-prepared Sn3(PO4)2 is in the range of a few micrometers. Figure 2 compares the potential and CV profiles of Sn3(PO4)2 in different potential ranges. The discharge profile contains two plateaus at ca. 1.80 and 0.90 V, respectively, and a slope between 0.80 and 0.00 V. When the cell is cycled between 0.00 and 3.00 V, it delivers an initial discharge capacity of 1200 mAh g-1 with a coulombic efficiency of 32.8 %. In contrast, the coulombic efficiency increases to 52.4 % when the cut-off discharge potential is set to 1.55 V (Figure 2a) in spite of a lower capacity. Moreover, the charge curves are totally different when the cell is cycled in different potential ranges. In the range of 0.00-3.00 V, the charge profile shows a


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plateau at ca. 0.45 V and then the potential rise rapidly to 3.00 V. In contrast, when the cell is discharged to 1.55

Figure 1. XRD patterns of Sn3(PO4)2 at different states: (a) as-prepared, (b) discharge to 1.55 V, (c) discharge to 0.00 V, (d) discharge to 1.55 V and then charge to 3.00 V, (e) discharge to 0.00 V and then charge to 3.00 V, (f) the XRD pattern of the protecting film.


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Figure 2. Initial potential profiles (a) and CV curves (b) of Sn3(PO4)2 within different potential ranges. V, the charge profile contains two plateaus at ca. 2.20 and 2.60 V, respectively. These differences can be seen more obviously on CV curves in corresponding potential ranges, as shown in Figure 1b. While there is only one anodic peak at ca. 0.50 V between 0.00 and 3.00 V, two anodic peaks appear in the range of 1.55-3.00 V. Furthermore, according to the integrated area, the coulombic efficiency grows from 44.1 % to 76.4 % as the cut-off discharge potential increases from 0.00 to 1.55 V. The above results reveal that Sn3(PO4)2 has different lithium storage mechanisms when it is discharged to different potentials. In addition, the potential dip at the beginning of discharge is believed to be related to the nucleation of metallic Sn, which will be discussed in the following section. The XRD patterns of Sn3(PO4)2 reveal that metallic Sn (JCPDS No.86-2264) is produced when the electrode is discharged to 1.55 V (Figure 1b). After the cell is recharged to 3.00 V, the diffraction peaks of metallic Sn disappear but no Sn3(PO4)2 is detected (Figure 1d). As the aggregation of the Sn particles becomes severe above 1.30 V,18 it is reasonable to believe that no detectable Sn exists in the sample at this state. When Sn3(PO4)2 is discharged to 0.00 V, the diffraction peaks of Li22Sn5 (JCPDS No.18-0753) appear while those of metallic Sn disappear (Figure 1c), confirming the formation of Li-Sn alloy. It is noteworthy that there is a large amount of metallic Sn in the fully discharged and then recharged sample (Figure 1e). No diffraction peaks of Sn3(PO4)2 are observed, in good agreement with the previous reports.17-18 As XRD is sensitive to the crystalline phase and no diffraction peaks of any species other than metallic Sn are detected, the recharge product should be disordered and need to be further characterized.


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According to the above results, it is believed that the reactions in the charge process are totally different when the material is cycled in different potential ranges. The above XRD results supply limited information about the conversion products, so XPS analysis was conducted to determine the chemical states of tin in Sn3(PO4)2 at different discharge/recharge states. The peak observed at 485.8 eV in the Sn3d spectrum of the as-prepared Sn3(PO4)2 (Figure 3a) is characteristic of Sn2+.23 After the material is discharged to 1.55 V, the main peak shifts to 485.3 eV, implying the formation of metallic Sn.24 Meanwhile, a new component appears at 483.8 eV belonging to LixSn (0

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