Mercaptopropionic acid-capped wurtzite Cu9Sn2Se9 nanocrystals as

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Mercaptopropionic acid-capped wurtzite Cu9Sn2Se9 nanocrystals as high performance anode materials for lithium-ion batteries Yue Lou, Min Zhang, Chunguang Li, Cailing Chen, Chen Liang, Zhan Shi, Dong Zhang, Gang Chen, Xiao-Bo Chen, and Shouhua Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14527 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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ACS Applied Materials & Interfaces

Mercaptopropionic Acid-capped Wurtzite Cu9Sn2Se9 Nanocrystals as High Performance Anode Materials for Lithium-ion Batteries Yue Lou, †,+ Min Zhang, ††,+ Chunguang Li, † Cailing Chen, † Chen Liang, † Zhan Shi,*, † Dong Zhang, *, ††

†

Gang Chen††, Xiao-Bo Chen, †, # and Shouhua Feng†

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin

University, Changchun 130012, P. R. China ††

Key Laboratory of Physics and Technology for Advance Batteries (Ministry of Education), College of

Physics, Jilin University, Changchun 130012, P. R. China # School of Engineering, RMIT University, Carlton, VIC 3053, Australia *Corresponding authors. E-mail: [email protected] and [email protected]. [+] These authors contributed equally to this work. KEYWORDS: metal selenide, Cu9Sn2Se9 nanoparticles, ligand engineering, Sn-based anode material, lithium-ion battery.

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ABSTRACT

In this research, we provide a simple but sound solution to address the low performance of lithium-ion batteries through preparation of wurtzite Cu9Sn2Se9 nanoparticles with uniform size distribution and morphology via a hot injection colloidal approach as a promising anode material. The Cu9Sn2Se9 nanoparticles anode exhibits superior rate performance and high reversible capacity of 979.8 mAh g-1 in the 100th cycle at a current density of 100 mA g-1, which is approximate 2 times of reported Cu-Sn-S framework (563 mA g-1), 1.5 times of reported pristine Cu2SnS3 (621 mA g-1) and comparable or higher than a number of reported Sn-based nanocomposites based anodes for lithium-ion batteries at the same cycle. The study demonstrate such outstanding properties are attributed to the high structural flexibility of the metal selenide and increased electronic connectivity by colloidal quantum dot ligand exchange procedure associated with mercaptopropionic acid (MPA). In addition, unlike most metal sulfides or selenides, it possesses a step-wise intercalation mechanism during the lithiation /delithiation cycles which is beneficial to buffer against volume variation of the alloy electrode materials. Such findings provide a new and feasible insight to guide the design and manufacturing of high performance lithiumion batteries for a broad variety of engineering applications.

Introduction: With the fast-growing energy demand, lithium-ion batteries (LIBs) have been recognized as a remarkable energy storage device during the past decades, owing to their high energy density, long cycle lifespan, cost-effectiveness and sustainable supply.1-4 However, low capacity (theoretical 372 mAh g-1) of commercial graphite, the prevalent anode material of LIBs, limits their large-scale commercialization for electric devices, vehicles and regional power grids.5 As such, it is of paramount importance to address this issue through development of alternative electrode materials with high specific capacity, stable cycling capability and high rate capability to yield high performance LIBs.6


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Of the existing anode materials, Sn-based nanocomposites (such as SnS/SnS2, SnSe2, SnO2, Sn-C etc.)718

outperform their competitors owing to their eco-friendliness, well-suited discharge potential and high

theoretical capacity derived from a conversion-alloying reaction. Sn-based anode materials are generally required to be constructed in a form of nanocomposites for enhanced electron transport and electrode/electrolyte contact area, and minimum volume expansion/shrinkage.16-28 Such a preparation technique is consisted of complicated preparation procedures and thus costly, which significantly blocks the pathways to commercialization of Sn-containing nanocomposites anode. In contrast, when monophasic Sn-based nanomaterials are employed as anode for LIBs, poor conductivity and large volume changes (up to ca. 260%) are developed with the progress of the lithium alloying-dealloying reactions. In addition, the anodes suffer from mechanical fracture and pulverization, which is attributed to the agglomeration of Sn-based nanoparticles (abbreviated as NPs hereafter) during the cycle process, therefore exhibiting poor long-term cycling stability and low rate performance.17,18 As such, it remains a challenge to tackle the existing bottleneck associated LIBs for large-scale applications through the design and development of pristine monodisperse nanocrystals-based anode materials with superior Li storage performance via a simple and practical approach. In this article, we propose to design and develop new anodes consisting of non-composite anode nanomaterials with uniform morphology and high conductivity to build LIBs with high electrode capacity, excellent rate performance, superior stable cycling capability, and more importantly, reasonable production cost. We focus on the development of promising colloidal synthesis strategy for monodisperse ternary selenide nanomaterials with delicate controls over their morphology and chemical compositions.29 Regarding electrode materials in battery research, various nanomaterials with superb size-, shape-, and compositional-tunability using colloidal synthesis method seemed to be encouraging candidates, since (a) Compositional-tunability makes doping inactive buffering components in intermetallic alloys convenient. (b) Uniform distribution and refinement of the size of nanocrystals can withstand volumetric changes to a great magnitude. (c) The protective capping molecules at the surface of NPs ensure mono-dispersion of NPs, and generate create ample area for further surface ACS Paragon Plus Environment


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modification.30 Precisely engineered NPs facilitate the minimum volume variation and high conductivity. (d) Such mature preparation procedure, i.e. colloidal synthesis, is a potent way to prepare various metal sulfides or selenides with accurate controls over their structures and compositions, though rare studies in this regard exist in the open literature. Herein, wurtzite Cu9Sn2Se9 (designated as CTSe hereafter) NPs with uniform morphology and a narrow size distribution are designed and synthesized by a simple colloidal synthesis method. Subsequently, MPACTSe NPs are achieved through colloidal quantum dot (CQD) ligand engineering process and employed as the major constituent of anode materials in a LIB device for characterizations. The process of CQD ligand engineering precisely enhances the conductivity of the anodes consisting of MPA-CTSe NPs, which imposes a substantial effect on the optimization of the electrochemical performance of the LIB device. Electrochemical measurements demonstrate that as a new anode material, MPA-CTSe NPs deliver an outstanding reversible capacity of 979.8 mAh g−1 at a current density of 100 mAg−1 after 100 cycles. Such specific capacity of MPA-CTSe NPs is superior to those of reported Cu-Sn-S framework (563 mA g-1), pure Cu2SnS3 based anodes (621 mAh g−1), commercial graphite (330 mAh g−1) and comparable or higher than a number of reported Sn-based nanocomposites, indicating a great potential for the design and production of next generation LIBs.

Experimental: Chemicals: Diphenyl diselenide (C12H10Se2, 98%), tin chloride dehydrate (SnCl2), mercaptopropionic acid (MPA) and oleic acid (OA, 90%) were obtained from Alfa Aesar. Oleylamine (OAm, 70%) was supplied by Sigma-Aldrich. Copper chloride (CuCl), ethanol and toluene were purchased from Shanghai Chemical Reagents Company. All reagents were used as received.


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Preparation of diphenyl diselenide /oleylamine solution: In brief, diphenyl diselenide (0.5 mmol) was added to and stirred in OAm (5 mL) for 3 h at room temperature under nitrogen atmosphere, to yield a bright orange solution. Then, the solution was transferred into a glass vial and stored in oven at 70 °C. Synthesis of wurtzite Cu9Sn2Se9 nanocrystals: In a typical synthesis, 0.15 mmol of CuCl and 0.075 mmol SnCl2 were mixed with 8 mL OAm and 2 ml OA in a 50 ml three-necked flask fitted with a reflux condenser. The mixture was purified alternatively in vacuum and nitrogen atmosphere for three times to eliminate adventitious water and dissolved oxygen. Subsequently, the system was heated up to 230 °C and followed by injection of 1.2 mL of diphenyl diselenide /oleylamine solution via syringe. The reaction proceeded at 240 °C for 6 min with stirring. After cooling down to room temperature, the asprepared products were washed with ethanol and toluene for three times and finally dispersed into toluene for further characterization. Ligand exchange procedure of OAm,OA-Capped Cu9Sn2Se9 nanocrystals: MPA-ethanol solution was prepared through mixing 0.6 mL of MPA, 0.5 mL of 30% NaOH aqueous solution and 1 mL of ethanol. Then it was transferred into 5.0 mL toluene solution containing 0.1 mmol Cu9Sn2Se9 nanocrystals. Ultrasounic bath was employed to transfer the nanocrystals from the superincumbent toluene into the underlying solution. MPA-capped Cu9Sn2Se9 nanocrystals were precipitated through addition of 3.0 mL of acetone and collected by centrifugation. Finally, the products were washed with double distilled water and acetone for three times and dried in vacuum oven over 12 h. Material characterization: Crystallographic characteristics of the nanomaterials were determined by Xray diffraction (XRD) on a Rigaku D/max-2500 diffractometer with Cu-Kα radiation operated at 200 mA and 40 kV. Morphology of Cu9Sn2Se9 nanoparticles were examined through transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai G2STwin with a field emission gun operating at 200 kV). Elemental analysis of the nanomaterials was carried out with a JEOL JSM-6700F microscope operated at 10 kV, equipped with an energy dispersive spectrometer (EDS). X-ray photoelectron spectroscopy (XPS) was recorded on ESCALAB 250 with Mg ACS Paragon Plus Environment


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Kα as X-ray source. The surface structures of OAm, OA-Capped and MPA-Capped Cu9Sn2Se9 nanocrystals were determined by infrared spectroscopy (IR) (IFS-66V/S, Bruker, Germany). Electrochemical characterization was conducted on CR2032-type coin cells using a piece of metallic Li foil as the reference electrode. The anode was composed of 70 wt.% active material, 20 wt.% active carbon, and 10 wt.% carboxymethyl cellulose binder (CMC), which was pasted on a Cu current collector and then cut into square shape with an area of 0.64 cm2 and dried in vacuum oven at 120 °C for 12 h. Loading mass of the active material was 1.5 ± 0.3 mg cm-2. The cathode and anode were separated by a glass fiber filter (Whatman GF/C). The electrolyte was prepared through mixing 1 M LiPF6 in a solution consisting of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1, by volume ratio) and 5 wt.% fluoroethylene carbonate (FEC). Galvanostatic charge-discharge was performed on a LAND-2010 automatic battery tester within a voltage window of 0.01-3.0 V. Electrochemical impedance spectroscopy (EIS) was performed on a Bio-Logic VSP multichannel potentiostatic-galvanostatic system. Impedance data were recorded at 5 mV (ac voltage) over a frequency range from 1 MHz to 1 mHz.

Results and discussion: Herein, pure wurtzite phase CTSe nanocrystals with well-controlled particle size and dispersion were prepared using a hot injection colloidal approach, where diphenyl diselenide was employed as the source of Se. XRD pattern (Figure 1a) reveals that the resultant CTSe NPs exhibit a similar crystal structure to that of previously reported Cu2SnSe3 metastable wurtzite structure, but contains vacancies.31 The distinct diffraction peaks of 2θ at 25.85, 26.80, 29.20, 37.60, 45.34, 48.94, 53.60,54.78, 61.02 and 68.96 (°) can be indexed to (100), (002), (101), (102), (110), (103), (112), (201), (202) and (203) crystallographic planes of the wurtzite structure with P63mc space groups. The lattice constants calculated from the experimental diffraction pattern of CTSe nanocrystals are a = b = 3.996 Å and c = ACS Paragon Plus Environment

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6.626 Å, respectively. Crystal structure of wurtzite CTSe NPs projected along [110], [100]/[010] and [001] are depicted in Figure 1b, 1c and 1d, respectively. Here, certain kinetic controls are the key to the preparation of metastable wurtzite CTSe NPs.32 Long alkyl chains ligands of oleic acid and oleylamine molecules at surface of the metastable nanocrystals can greatly reduce surface free-energy, which is beneficial to stabilize metastable phases. Highly active diphenyl diselenide can trigger rapid nanocrystal growth towards the formation of metastable kinetic products, which plays a key role in extendable preparation of Cu-based WZ-derived multinary chalcogenides.33 TEM examinations were employed to reveal the size and morphology of the as-synthesized CTSe NPs (Figure 2a and 2b), where hexagon particles with a mean diameter of 32 nm were uniformly dispersed. The HRTEM image (Figure 2c) of a randomly-selected CTSe NPs illustrates that the crystal lattice fringes with d-spacing of 0.347 nm correspond to the reflection of (100) planes, indicating a highly crystalline nature and wurtzite structure of the CTSe NPs. The fast fourier transform (FFT) of a selected area (inset of Figure 2c) reveals a characteristic hexagonal structure as viewed along the c-axis of CTSe NPs.34 Elemental composition of the nanocrystal yields was analyzed by SEM equipped with EDS. The EDS spectrum of a field of nanocrystals (Figure 2d) confirms Cu/Sn/Se ratio is close to 9:2:9, which confirms the presence of spatial vacancies in the crystal structure.34,35 STEM-EDS mapping images (Figures S2) of CTSe NPs reveal the distribution of Cu, Sn, and Se elements, verifying the presence of uniformly alloyed as-synthesized CTSe NPs.36 In addition, oxidation state of CTSe NPs was investigated by means of XPS. XPS survey spectra (Figure 3) of the CTSe NPs identify the presence of Cu, Sn and Se. Two characteristic binding energy peaks of Cu 2p appeared at 932.1 and 951.7 eV with a splitting of 19.6 eV. Two peaks centered of Sn 3d core level spectrum existed at 486.2 eV and 494.6 eV. Se 3d peaks were located at 54.4 eV, which can be assigned to the presence of Se species. These binding energies are consistent with the reported XPS analysis of CTSe NPs with offset less than 0.2 eV, which demonstrate that the CTSe NPs were yielded. The difference in binding energy suggests the existence of vacancies.33,37 Size of the tunnels of wurtzite CTSe NPs with lattice defects is greater than the radius of Li ions (1.36 Å), which could accommodate the moving Li ions through diffusions. The existence of ACS Paragon Plus Environment


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vacancies can provide additional Li storage and thus optimize the performance of LIBs.38 Monodispersed nano-materials can minimize volume expansion, and alleviate the mechanical stress induced by the volume changes and the peeling of the active materials during a long-run chargingdischarging cycling process. It should be noted that doping inactive buffering components in crystal lattice, such as Cu in this instance, can increase electronic conductivity to a great degree.39 As such, the as-synthesized ternary wurtzite CTSe NPs can be an ideal material candidate for anode of Li-ion batteries, attributed to their crystal structure, refined size, unique morphology, and favorable chemical composition. As anticipated, the presence of highly isolating oleic acid and oleylamine (abbreviated as OAm and OA hereafter) capping layer deteriorate the electrical conductivity of the electrode material for Li-ion storage devices, which is another key attribute to the performance of anode materials. We therefore employed a capping exchange approach, i.e. a solution-phase ligand exchange method, for replacing the long alkyl chains ligands (OAm and OA) with short ligands at the surface of CQD surface with the aid of mercaptopropionic acid (MPA). As illustrated in Figure 4a, CTSe NPs solution were ultrasounically mixed with MPA-ethanol solution, which leads to rapid and complete migration of CTSe NPs in the polar solvent, i.e. toluene. After the ligand exchange process, size of the CTSe NPs were reduced (Figure 4b), which is attributed to the etching effect of NaOH. The monodispersity of the CTSe NPs can be maintained even when the equilibrium distance of adjacent particles was shortened, which can buffer against volume expansion, and enhance the conductivity of the yielded CTSe NPs. Crystal structure of CTSe NPs after ligand exchange procedure, designated as MPA-Capped CTSe NPs, was characterized by XRD. XRD pattern (Figure 4c) demonstrates that original crystal structure is retained. Moreover, as shown in Figure 4d, IR spectroscopy was employed to determine the capping agents binding to the surface of the CTSe NPs. Black line indicates the IR spectra of OAm, OA-Capped CTSe NPs and red line corresponds to MPA-Capped CTSe NPs, respectively. The peak at 3340 cm-1 is attributed to N-H stretching and bending modes, which indicates that OAm is the main ligand bonded to CTSe nanocrystals. After ligand-exchange with MPA, it is apparent that S-H vibration absorption near 2600 ACS Paragon Plus Environment

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cm-1 does not exist (red line), confirming the formation of S-Cu/S-Sn between the CTSe NPs and MPA. Absence of the peak at 3340 cm-1 and presence of two new peaks at 1566 and 1403 cm-1 corresponding to characteristic absorption peak of COOH of MPA, further indicate the substitution of capping agents OAm with MPA.37 In addition, elemental composition of MPA-capped CTSe NPs was characterized by EDS spectrum. It (Supporting Information, Figure S1) reveals that the addition of sulfur by using MPA and the loss of Sn and Se which caused by alkali etching of nanoparticles may further increase lattice defects. 2032 coin-type cells consisting of Li metal as counter electrode, CTSe NPs as anode, and 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 vol. ratio) and 5 wt.% fluoroethylene carbonate (FEC) as electrolyte, were assembled to explore the electrochemical Li insertion and extraction behavior. Cyclic voltammograms (CV) curves (Figure 5a) were plotted over a potential range of 0.01-3.0 V with a scan rate of 0.1 mV s-1 at 1st, 2nd, 5th, 10th cycles, respectively, to elucidate the electrochemical redox process associated with the MPA-capped CTSe NPs anode containing cells. The electrochemical redox reactions of the MPA-CTSe NPs anodes differs from those of the reported Snbased ternary sulphides.40-43 In brief, four distinct reduction peaks centered at 1.99 V, 1.67 V, 1.60 V and 1.45 V were observed in the cathodic wave during the first cycle scan, which demonstrates the formation of LixCu9Sn2Se9 and the solid electrolyte interphase (SEI). The corresponding anodic curve exhibits a weak but broad oxidation peak at 0.54 V and two prominent peaks at 1.88 V and 2.22 V, which can be assigned to the delithiation process of LixCu9Sn2Se9 and the decomposition of SEI. In the subsequent cycles (i.e. 2-10), a broad cathodic peak between 0.2 and 0.8 V and an anodic oxidation peak at 2.06 V emerge, signifying phase transformations during the (de)lithiation process of CTSe NPs and the reversible formation/decomposition of solid electrolyte interphase (SEI). Further evidence is provided in the following discussion regarding XRD, TEM and EIS results acquired at the 1st, 2nd and 100th cycles, respectively. In addition, the reproducibility of CV plots recorded at subsequent cycles indicates promising stability of the reversible electrochemical processes. In contrast, regarding OAm,OA-capped CTSe NPs anode counterparts (Supporting Information, Figure S3a), CV curves are ACS Paragon Plus Environment


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consisted of comparative peaks to those of MPA-capped CTSe NPs anode, evidencing similar electrochemical reactions. However, it is discernable that the intensity of all cathodic and anodic peaks decreases steeply after ten cycles, which indicates poor cycling stability. Galvanostatic charge and discharge curves of MPA-capped CTSe NPs anode for LIB (Figure 5b) were recorded for the selected cycles at 100 mA g−1 over a potential range of 0.01 and 3.0 V (vs. Li+/Li). The observed voltage plateaus match well to the redox peaks in the CV curves (Figure 5a). The initial discharge and discharge capacities are 564.1 and 466.6 mA h g-1, corresponding to the initial coulombic efficiency of 78.8%. The loss of irreversible capacity may be attributed to the formation of SEI film.6 The charge-discharge profiles agree well after the initial capacity which demonstrates the excellent cycle stability of the MPA-capped CTSe NPs electrode. For comparison, the charge and discharge curves for OAm,OA-capped CTSe NPs anode counterparts for the similar cycles are provided as Supporting Information (Figure S3b), which demonstrates a sharp reduction in the charge and discharge capacity. The cycling stability of MPA-capped and OAm,OA-capped CTSe NPs anodes at a current density of 100 mA g−1 between 0.01 and 3.0 V vs. Li+/Li are presented in Figure 5c and Figure S3c, respectively. It is noted that the discharge capacity of MPA-capped CTSe NPs progressively increases from 564.1 mA h g-1 and stabilizes at 979.8 mA h g-1 after 100 cycles at 100 mA g-1 and the corresponding coulombic efficiency is higher than 97% after initial several cycles. Comparative increases in discharge capacity have been reported previously, which is ascribed to the activation of active materials, improvement in electrical conductivity or reversible formation/decomposition of the SEI film.16, 32 On the contrary, the discharge capacity of OAm,OA-capped CTSe NPs anode suffers

a significant

degradation from 597.4 down to 187.8 mA h g−1 at 100 mA g−1 after 100 cycles, which is strong evidence of inferior charge and discharge capacity, and cycling stability (Figure S3c). The study of rate capability of MPA-capped CTSe NPs anode for LIB reveals excellent capacity retention with stepwise-increasing current density from 100 to 5000 mA g−1 (Figure 5d). Marked ACS Paragon Plus Environment

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discharge capabilities of 576.5, 493.7, 418.0, 316.5, and 170.7 mAh g−1 are yielded 200, 500, 1000, 2000 and 5000 mA g−1, respectively. It is remarkable that a high and stable discharge capacity of 1008.8 mAh g−1 can finally be attained when the current density returns to 100 mA g−1 after the rate capability test. Such a high tolerance to high-rate cycles is derived from the high structural flexibility of MPAcapped CTSe NPs anode.44 As a comparison, OAm,OA-capped CTSe NPs anodes exhibit initial reversible capacities about 640 mAh g−1 at 100 mA g−1 and only retained 59.7 mAh g−1 at 5000 mA g−1 (Supporting Information, Figure S3d). In addition, discharge capacity only returns to 210 mAh g−1 when the cycling current is back to 100 mA g−1, indicating inferior rate performance of OAm,OA-capped CTSe NPs anode. To unveil the mechanism regulating the cycling stability of MPA-CTSe NPs based anode, MPACTSe NPs were retrieved from the cells after discharging/charging cycles to different potentials, in glovebox under protection of N2 atmosphere for characterisations. Ex-situ XRD patterns (Figure 6) were performed to reveal the phase transition of the MPA-CTSe NPs as a function of potentials during discharge/charge processes (i.e. 1st, 2nd, and 100th). It is apparent that no phase transition takes place during the initial discharge of the cells, indicating the insertion of Li+ ions imposes a negligible impact on crystallographic structure of MPA-CTSe NPs. Subsequently, an irreversible phase change between wurtzite and cubic CTSe NPs occurs during the first charging process to 1.9 V. The as-synthesized wurtzite MPA-CTSe NPs are multicomponent selenide with metastable kinetics. When the delithiation process occurs, the crystallographic structure of CTSe NPs rearranges and thermodynamically stable cubic structure tends to form. For the next discharge/charge processes, a reversible process proceeds, leading to the retaining cubic CTSe phases for Li storage. Thus, Li+ ions should be inserted into the space between CTSe lattices through an step-wise intercalation mechanism during the following cycles, which is not seen in most existing Sn-based alloy electrodes. Step-wise take-up of Li+ can be assumed according to the following reaction (x, y ≥0) wurtzite CTSe + x Li+ + xe−

wurtzite LixCTSe

(1)



wurtzite LixCTSe cubic CTSe + yLi+ + ye−

cubic CTSe + xLi+ + xe− cubic LiyCTSe

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(2) (3)

The evolution of morphology and crystallographic structure of MPA-CTSe NPs after the initial lithiation (charge) and delithiation (discharge) processes were further examined by ex-situ TEM (Figure 7a and 7d). The original morphology of MPA-CTSe NPs is maintained after the first discharge cycle to 0.01 V. Both HRTEM (Figure 7b) and FFT (Figure 7c) demonstrate the initial wurtzite structural characteristics of MPA-CTSe NPs. As a comparison, after first charge cycle to 3.0 V, MPA-CTSe NPs tend to exhibit a relatively irregular morphology. Corresponding HRTEM image (Figure 7e) and FFT (Figure 7f) demonstrate clear lattices of 0.206 nm and 0.336 nm, which index to (220) planes and (1-11) planes of cubic CTSe.35 Moreover, in the Supporting Information (Figure S4a and S4b), it is obvious that the size of cubic MPA-CTSe NPs after 100 cycles is smaller than that of initial MPA-CTSe NPs prior to charge and discharge cycles. The crystal lattice fringes with a d-spacing of 0.336 nm correspond to the reflection of (111) planes, indicative of high crystallinity of cubic CTSe NPs. The selected-area electron diffraction (SEAD) pattern (Supporting Information, Figure S4c) confirms the cubic structure of CTSe NPs further. The ex-situ data reported prove MPA-CTSe NPs possess an step-wise intercalation mechanism during the lithiation /delithiation cycles, not for conversion-alloying reaction which are conceptually novel. From a molecular perspective, the reason may include (1) the spatial preconditions are advantageous owing to relatively large size of tunnels of wurtzite CTSe NPs with lattice defects (2) the interaction of the entering hard lithium ions with the soft hosting chalcogenide anions not too strong.44 To further unveil to the underling mechanism for high capacity of MPA-CTSe NPs anode, i.e. the correlation between extra lithium interfacial storage and the reversible formation/decomposition of the SEI film, the LIB devices were analyzed through EIS at ac potential of 5 mV spanning a frequency range of 1 MHz to 1 mHz after one, two and 100 cycles . The obtained EIS data were fitted through an equivalent circuit (Figure S5) model of the studied system and the fitting parameters are provided in ACS Paragon Plus Environment

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Table 1. Two primary components are apparent, i.e. a semicircle at high-frequency and a straight line at low frequency regions, respectively (Figure 8). RS, RF and RCT represent the internal resistance, the contact resistance by the formation of the SEI layer and the charge-transfer resistance, respectively. As shown in Figure 8a, one semicircle is detected in the open circuit voltage (OCV) state which can be attributed to a charge-transfer process. Two semicircle emerges after being discharged to 0.01 V and the semicircle in the medium frequency region disappears after the electrode was recharged to 3.0 V during first and second charge/discharge cycling test, indicating the reversible formation/decomposition of the SEI film. In addition, according to the Nyquist impedance plots (Figure 8b) after 100 cycles, the chargetransfer resistance of MPA-CTSe NPs anode is 67.2Ω, much smaller than that 116.7Ωof the electrode after the first cycle at 100 mA g−1, which indicates that the electron transfer and the diffusion of Li+ ion are improved, and contribute greatly to the superior rate capability and excellent capacity retention.23 The abovementioned results elucidate that the increasement of electronic conductivity and reversible formation/decomposition of the SEI film occur during the cycle process. Such excellent electrochemical performance of MPA-capped CTSe NPs is one of the best materials among the reported pristine Sn-based nanocrystal electrodes and even comparable to a great variety of Sn-based hybrid electrodes. Table S1 in Supporting Information presents a detailed comparison of the electrochemical performance between the MPA-capped CTSe NPs electrodes of this article and some representative Sn-based electrodes reported recently. The step-wise intercalation mechanism during the lithiation/delithiation cycles is able to significantly accommodate volume change during reduplicative Li+ insertion and extraction, thus eliminating the pulverization problem and improving the structural integrity during cycling. Additionally, the superior electrochemical performance of MPA-capped CTSe NPs can also be related to encourage charge transfer accompanied by successful ligand exchange and doping of inactive buffering components Cu.45 Moreover, uniform monodisperse nanoparticles can adequately contact with the electrolyte which facilitate the diffusion of Li ion. Hence, we believe that the reported preparation strategy and surface modification method could provide insight into design and ACS Paragon Plus Environment


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manufacture of new electrode materials with uniform size, morphology and optimized electrochemical performance for lithium-ion batteries. Conclusions: In summary, ternary CTSe NPs with a wurtzite structure have been synthesized using a hot injection colloidal synthesis method, followed by a ligand exchange procedure with MPA. The surfactant-assisted functional process is able to compact geographic arrangement of the CTSe NPs and improves the overall electrical conductivity of the resulting anodes. A novel electrochemical reaction mechanism of Sn-based electrodes is established and investigated, which is reasonable resulting in a high specific capacity and excellent rate capacity. The MPA-capped CTSe NPs based anodes yield a reversible capacity of 979.8 mA h/g at a current density of 100 mA g-1 after 100 charging-discharging cycles, and high rate capacity, which outperforms a number of Sn based composites in open reports. The proposed synthesis strategy, i.e. colloidal synthesis followed by ligand exchange engineering, could be applicable to a great diversity of multi-component metal selenides with delicate controls over crystal phases, composition, size, and morphology to guarantee high electrochemical performance for future LIBs.

Figure 1. (a) Experimental (upper black line) and simulated (bottom red spikes) XRD patterns of wurztite CTSe NPs. (b-d) crystallographic structure of wurtzite CTSe NPs. (Green pyramids are Cu/Sn and red round dots stand for Se).


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Figure 2. (a-c) TEM and HRTEM micrographs, and (d) EDS spectrum and elemental concentrations of quasi-hexagonal CTSe NPs. Inset of (c) is selected-area FFT.

Figure 3. XPS spectra of as-synthesized CTSe NPs of (a) Cu 2p, (b) Sn 3d, and (c) Se 3d core levels.

Figure 4. (a) A photographic illustrating the phase transfer of CTSe NPs induced by ligand exchange process. (b) TEM image of the CTSe NPs after ligand exchange. (c) XRD pattern of ACS Paragon Plus Environment


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MPA-capped CTSe NPs. Red line indicates the simulated powder XRD pattern. (d) IR spectra of CTSe NPs before (black) and after (red) ligand exchange.

Figure 5. (a) CV curves of MPA-capped CTSe NPs anode recorded with a scan rate of 0.1 mV s−1. (b) Galvanostatic charge/discharge profiles of MPA-capped CTSe NPs for selected cycles between 0.01 and 3 V (vs Li+/Li) at 100 mA g−1. (c) Cycling stability/retention and corresponding coulombic efficiency of MPA-capped CTSe NPs anode at 100 mA g−1. (d) Rate capability of MPA-capped CTSe NPs from 100 to 5000 mA g-1.


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Figure 6. Ex-situ XRD patterns of MPA-CTSe NPs collected from the cells after the first, second and 100th discharge/charge processes. Bottom spikes indicate characteristic XRD peak positions of wurtzite and cubic CTSe NPs.


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Figure 7. a) TEM, b) HRTEM and c) the corresponding Fourier filtering images of MPAcapped CTSe NPs after being initially discharged to 0.01 V. d) TEM, e) HRTEM, and f) their corresponding Fourier filtering images of MPA-capped CTSe NPs after being initially recharged to 3.0 V.


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Figure 8. (a) Nyquist plots of MPA-capped CTSe NPs electrode at different states of the electrochemical process. (b) Nyquist plots of the fully delithiated MPA-capped CTSe NPs electrode after 100 cycles.


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Table 1. Fitted impedance parameters of MPA-capped CTSe NPs at different states of the electrochemical process. Cycle number

Rs [Ω]

Rf [Ω]

Rct [Ω]

OCV

3.4

–

573.7

1st discharge

4.5

198.2

236.6

1st charge

4.2

–

116.7

2nd discharge

4.4

200.7

190.5

2nd charge

4.2

–

101.7

100th charge

23.4

67.2

ASSOCIATED CONTENT Supporting Information. EDS spectrum of CTSe NPs after ligand exchange procedure. CV curves, Galvanostatic charge/discharge profiles, Ccling stability/retention and corresponding Coulombic efficiency and Rate capability of OAm,OA-capped CTSe NPs anode. TEM image, HRTEM image and SAED pattern of the MPA-capped CTSe NPs electrode material after 100 cycling. The equivalent circuit used for fitting anode impedance spectra. Comparison on Electrochemical Properties of Our Work and Other Reported Sn-based Electrodes.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]. Tel/Fax: +86-431 85168662.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of China (Grant No. 21621001 and 21371069), the National Key Research and Development Program of China (Grant No. 2016YFB0701100), the 111 project (No. B17020) and the S&T Development Program of Jilin Province of China (No. 20160101325JC,) and “973” project (No. 2015CB251103). X.C. and Z.S. gratefully acknowledge the financial support from the Natural Science Foundation of China through Research Fund for International Young Scientists scheme (21550110190).

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Mercaptopropionic acid-capped wurtzite Cu9Sn2Se9 nanocrystals as

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