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Controllably Electrochemical Synthesis of Copper Sulfides as Sodium Ion Battery Anodes with Superior Rate Capability and Ultra-long Cycle Life Haomiao Li, Kangli Wang, Shijie Cheng, and Kai Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19138 • Publication Date (Web): 09 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018
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Controllably Electrochemical Synthesis of Copper Sulfides as Sodium Ion Battery Anodes with Superior Rate Capability and Ultra-long Cycle Life Haomiao Li,1,2 Kangli Wang*,1 Shijie Cheng1 and Kai Jiang*1,2 1
State Key Laboratory of Advanced Electromagnetic Engineering and Technology, School of
Electrical and Electronic Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, China 430074. 2
State Key Laboratory of Materials Processing and Die & Mould Technology, School of
Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, China 430074. KEYWORDS: electrochemical synthesis, copper sulfides, sodium ion battery anodes, high rate capability, long cycle life
ABSTRACT Sodium ion batteries (SIBs) are prospective alternative to lithium ion batteries (LIBs) for large-scale energy storage applications, owing to the abundant resources of sodium. Metal sulfides are deemed to be promising anode materials for SIBs due to low-cost and eco-friendliness. Herein, for the first time, series of copper sulfides (Cu2S, Cu7S4 and Cu7KS4) are controllably synthesized via a facile electrochemical route in KCl-NaCl-Na2S molten salts. The as-prepared Cu2S with micron-sized flakes structure is firstly investigated as anode of SIBs, which delivers a capacity of 430 mAh g-1 with high initial coulombic efficiency of 84.9% at current density of 100 mA g-1. Moreover, the Cu2S anode demonstrates superior capability (337 mAh g-1 at 20 A g-1, corresponding to 50 C) and ultra-long cycle performance (88.2% of capacity retention after 5000 cycles at 5 A g-1, corresponding to ACS Paragon Plus Environment
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0.0024% of fade rate per cycle). Meanwhile, the pseudo-capacitance contribution and robust porous structure in-situ formed during cycling endow the Cu2S anodes with outstanding rate capability and enhanced cyclic performance, which are revealed by kinetics analysis and ex-situ characterization.
1. INTRODUCTION Energy storage technologies are deemed to be the key of integrating intermittent renewable energy generation (such as wind and solar) into the grid.1-3As a promising energy storage device, sodium ion batteries (SIBs) have been prompting extensive attentions both in academic community and industry, owing to their abundant reserves of Na resource and competitive power/energy density. However, comparing with lithium ion batteries (LIBs), the radius of Na+ is 35% larger than that of Li+, which brings greater challenges to SIBs for searching suitable host materials to accommodating the larger Na+.4-6 In the past several years, tremendous efforts have been devoted to developing cathode materials for SIBs, such as polyanionic compounds7-10, layered oxides11-14 and Prussian-blue based materials15-17, providing numerous choices for SIBs. Nevertheless, the shortage of high performance anode materials has been stunting the development of SIBs since Na+ cannot intercalate into the commercialized graphite anode which is commonly used in LIBs. Even though the studies on anode materials, such as hard carbon-based materials18-22, transition metal oxide materials23-26 and metallic compounds27-31 are progressive in recent years, however, most of those materials are still suffering from low-capacity, poor cyclic performance or limited rate capability. Thus, developing anode materials with all merits of high capacity, excellent durability and superior rate capability is urgent, however with great challenge for SIBs. Metal sulfides, as promising anode materials, have been intensively investigated in SIBs in ACS Paragon Plus Environment
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recent years, due to the merits of low-cost, eco-friendliness and good conductivity.32 Several typical metal sulfides, such as SnS233-35, Sb2S336,37, Bi2S339,40, MoS241-43, NiS44 and FeS245-48, exhibit impressive Na+ storage performance when serving as anode of SIBs. Thereinto, one class of metal sulfides, incorporating both conversion and alloying reactions, could provide high Na+ storage capacity, for instance SnS2 (1136.8 mAh g-1), Sb2S3 (947.3 mAh g-1) and Bi2S3 (625.7 mAh g-1). However, the consequent severe volume change impedes the improving of their long-term cycle performance. For example, Qu et al.48 designed a layered SnS2@rGO composites via hydrothermal method, and demonstrated its rate performance (544 mAh g-1 at 2 A g-1) and cycle-life (84% of capacity retention after 400 cycles). While, the Na+ storage capacity of other kind of metal sulfides are mainly contributed from conversion reaction, such as MoS2, Co3S4[49], NiS and FeS2. The inert metal products formed during sodiation process are contributed to enhance the cycle stability by buffering the volume expansion and improving conductivity of electrodes. For example, Ren et al.41 reported a ultrathin MoS2 nanosheets@C composite as anode of SIBs, achieving 265 mAh g-1 of capacity after 1000 cycles at 1 A g-1. Pyrite FeS2 microspheres, reported by Chen et al.50, displayed excellent cycle stability (90% capacity retention for 20000 cycles with cut-off voltage of 0.8 V) and rate capability (170 mAh g-1 at 20 A g-1). Copper sulfides, a type of functional semiconductor with various stoichiometrics, have wide applications in the fields of sensors, solar cells and optoelectronic device.51, 52 Moreover, the merits of comparable band gap energy (Eg = 1.2 eV), excellent electronic conductivity (ca. 104 S cm-1), environmental friendliness and abundant reserves also endowed copper sulfides a promising prospective in application of energy storage batteries, such as LIBs and SIBs. Actually, typical copper sulfides (CuS and Cu2S) has been investigated in LIBs and shown ACS Paragon Plus Environment
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impressive Li+ storage performance.53-54 However, the major issue hindering the application of copper sulfides in LIBs is its poor cycle stability. Herein, a novel method of electrolysis anodes directly synthesis metal sulfides was developed in this work. In this process, different phase structures copper sulfides (Cu2S, Cu7S4 and Cu7KS4) were controllably electrochemically synthesized in molten salts which is a facile and controllable methods inspired by FFC process, widely used for preparation of metal/alloys55-57 and functional materials58-60. The as-prepared copper sulfides were investigated as anode of SIBs and demonstrated excellent Na+ storage performance. Especially, the Cu2S anodes with micron-sized flake structure exhibit superior capability (337 mAh g-1 capacity at 20 A g-1, less than 20% capacity fading comparing with initial capacity at 0.1 A g-1) and ultra-long cycle performance (88.2% of capacity retention after 5000 cycles at 5 A g-1). 2. EXPERIMENTAL SECTION Electrolysis set-up and materials: An electrolysis set-up (three-electrode for cyclic voltammetry test, two-electrode for electrochemical synthesis) was sealed in a stainless retort filled with following argon and heated in a vertical type tube furnace. The anode was consisted of a small graphite cup containing Cu pellets (dia. 10 mm, weight 1.0 g) which was pre-pressed from Cu powder at pressure of ~10 Mpa. A graphite rod (10 mm dia.) connected to a Mo wire was served as cathode. For CV testing, a Ag/AgCl reference electrode was fabricated by sealing a Ag wire (0.5 mm dia.) and ~ 5g mixture salts (3wt% of AgCl in NaCl-KCl eutectic) into an end-closed mullite tube (ID = 4 mm). High purity NaCl and KCl (> 99%, from Aladdin) were dried under vacuum at 120 oC for 12 h to remove residual water before used as electrolytes together with Na2S. 44 g NaCl and 56 g KCl were weighted respectively, mixed and transferred into an alumina crucible (ID 40 mm. depth 100 mm). The ACS Paragon Plus Environment
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cell was assembled and transferred into the stainless retort with continuous argon flow, then the assembly was initially held at 250 oC for 4h and then heated up to the working temperature of 700 oC. Electrochemical synthesis: The potentiostatic electrolysis was conducted at 700 oC by using the two-electrode cell described above. Before adding Na2S into molten salts as S source, a pre-electrolysis process at 2.2 V was performed with a graphite rod anode and steel wire cathode to remove the impurities from molten salt. After the current of pre-electrolysis stabilized at a small value (called background current, after about 2 h), took out the steel wire and added the pre-dried Na2S into the cell. Copper sulfides were synthesized by applying predetermined voltages between Cu anode and graphite cathode in molten salts. After the electrolysis is finished, the anodes were lifted out of the molten salts and cooled down in the following argon, and washed with deionized water, then dried at 60 oC over 12 h for further tests. Materials characterization: The phase structure, composition and valence state of samples were characterized by powder X-ray diffractions (XRD) and X-ray photoelectron spectroscopy (XPS). The XRD patterns were recorded on a PANalytical X’Pert PRO (Cu-K generator). The XPS survey was carried out on an AXIS-ULTRA DLD-600W spectrometer (from Kratos Corp., Japan). The morphologies of products were observed using a field-emission scanning electron microscopy (SEM, FEI Nova NanoSEM 450). The electrochemical performance of as-obtained products was characterized with 2025 type coin cells. The electrodes (diameter ~11 mm) to be measured were made by mixing the as-prepared materials (Cu2S) with Ketjen black and polyacrylic acid/carboxyl methyl cellulose (PAA/CMC = 1:1) in de-ionized water in the ratio 7:2:1. The mass loading of the ACS Paragon Plus Environment
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active material is 1.8~2.2 mg cm-2. The electrolyte is 1 M NaPF6 dissolved in ethylene glycol dimethyl ether (DME). Sodium foil and microporous glass fiber membrane (Whatman) were used as counter electrode and separator. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were characterized on PGSTAT302N potentiostat/galvanostat (AUTOLAB, Netherlands) and galvanostatic cycling tests were performed on LAND2001A battery tester (LANHE, China) in the voltage range of 0.01-2.5 V at room temperature. 3. RESULTS AND DISCUSSION Figure 1a describes the design of the three-electrode cell for cyclic voltammetry testing with Ag/AgCl reference electrode, graphite work electrode and counter electrode. Considering high solubility of Na2S and K2S in NaCl-KCl eutectic, NaCl-KCl-Na2S melts are selected as electrolyte as well as providing S source in this work. Figure 1b presents the CV curves of graphite electrode recorded in molten NaCl-KCl (-Na2S) at 700 oC. As can be seen, the electrochemical widow of molten NaCl-KCl is greater than 3.3 V. The reduction/oxidation peaks at ~-2.1 V and 1.2 V (vs. Ag/AgCl) are attributed to Na deposition/stripping and oxidation of Cl-, respectively. While, after adding Na2S into the cell, the broad peak around 0 V (vs. Ag/AgCl) in molten NaCl-KCl-Na2S is deemed as the oxidation of S2-, which is consistent with the previous report.61
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Figure 1. Schematic of the three-electrode cell (a) and cyclic voltammograms of the graphite electrode in molten KCl-NaCl (-Na2S) at 700 oC (b). Scan rate: 50 mV s-1.
The processes of electrolytic synthesis were carried out in the two-electrode cell described above, under different electrolysis voltages (0.6 V, 0.8 V and 1.0 V) sustaining 10 h, respectively. XRD patterns of as-prepared products under aforementioned voltages are displayed in figure 2a. Three stoichiometrics copper sulfides: Cu2S, Cu7S4 and Cu7KS4, were obtained respectively, according to adjusting electrolysis voltage from 0.6 V to 1.0 V. When the voltage below to 0.6 V, orthorhombic Cu2S (JCPDS No. 23-0961) with trace of Cu1.97S is detected by XRD, which is helpful to enhancing the conductivity of Cu2S.[62] While, orthorhombic Cu7S4 (JCPDS No. 22-0250) and tetragonal Cu7KS4 (JCPDS No. 47-1334) are obtained when the voltage increases to 0.8 V and 1.0 V, respectively. The morphologies of products obtained are presented in figure 2b-d. Micron-sized flakes with smooth surfaces are observed in the SEM images when the voltage below to 0.6 V (SEM images of product at 0.4 V shown in figure S1). The thickness of the flakes is several micrometers while the diameter exceeds 100 micrometers. However, a flower-like structure composed of hundred-nanometer plates attaching to the surface of flakes is formed when voltage is above 0.8 V. ACS Paragon Plus Environment
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Figure 2. XRD patterns (a) and SEM images (b-d) of products obtained under electrolysis voltage of 0.6 V, 0.8V and 1.0 V. The chemical state of the Cu2S obtained under 0.6 V is further characterized by X-ray photoelectron spectroscopy (XPS). The existence of C, Cu, O and S is proved in the survey scan spectrum shown in figure 3a, and impurities of C and O should be introduced during the preparation of samples. From the high-resolution XPS spectra shown in figure 3b, the peaks at 952.4 eV and 932.5 eV correspond to Cu 2p3/2 and 2p1/2 of Cu1+, while the small peaks around 954.7 eV and 934.6 eV are assigned to Cu2+ which may demonstrates the existing of other stoichiometry CuxS (1
