Interpreting the Abnormal Charge-discharge Plateau Migration in

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Interpreting the Abnormal Charge-discharge Plateau Migration in CuxS during Long Term Cycling Zuguang Yang, Ting Chen, Chunjin Wu, Jie Qu, Zhen-Guo Wu, Xiao-Dong Guo, Benhe Zhong, Huakun Liu, and Shi Xue Dou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18864 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

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ACS Applied Materials & Interfaces

Interpreting

the

Abnormal

Charge-discharge

Plateau

Migration in CuxS during Long Term Cycling Zuguang Yang,† Ting Chen,† Chunjin Wu,† Jie Qu,† Zhenguo Wu,*,†,§ Xiaodong Guo, *,†,‡

Benhe Zhong,† Huakun Liu,‡ Shixue Dou.‡

†

School of Chemical Engineering, Sichuan University, Chengdu 610065, China.

‡

Institute for Superconducting and Electronic Materials, University of Wollongong, Wollogong,

NSW 2522, Australia. §

State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Energy, Xiamen

University, Xiamen, 361005, China.

Abstract: Voltage polarization during cycling, the charge potential increase of anode or discharge plateau decrease of cathode, is widely observed and would lower the output voltage. Conversely, an anomalous potential plateau negative migration phenomenon was observed in CuxS anode of sodium ion battery. In this study, the behind mechanism was clarified from the switch of intercalation-conversion reactions and structure evolution. The dynamic cooperation between intercalation and conversion reactions may root the potential plateau negative migration during cycling. In the initial stage, the intercalation-type reaction with Na3Cu4S4 and Na4Cu2S3 products at 2.13 and 1.92 V would dominate the early migration process of potential plateaus. Second stage, the conversion-type reaction dominated with Na2S and metallic copper formed at 1.85 and 1.53 V in the later period. The aforementioned results would spot new sight on the electrochemical behavior of transition metal sulfide anode and provide a clue for reducing the voltage polarization. Keywords: sodium-ion battery; CuxS; long term cycling; potentials plateau migration; reaction mechanism evolution

1. Introduction The energy quality, how much useful power or product a unit of energy can provide, is crucial to electrochemical energy storage devices.1,2 Thus, not only a high specific capacity is required, but also cathode materials with high discharge voltage and a low charge voltage for anode materials are highly demanded.3-5 In general, the 1

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polarization of electrode during cycling would lead cathode discharge voltage decay and anode charge voltage raise, which result in lower energy density.6-10 Interestingly, an abnormal potential plateau negative migration during cycling could be observed in transition metal sulfides (TMSs), including FeS2, CuxS, and CoSe anode.11-13 Over the years, morphology design and nanostructure engineering, which aim to alleviate the volume expansion and shorten ions diffusion path, have been extensively investigated.14-31 Although a series TMSs with high energy density and long-life span have been prepared, the investigation of working potential plateau, which is crucial for battery performance, is relatively ignored.32-34 Recently, Li et al. reported that CuS would react with Na+ to form the intermedia NaαCuβSγ firstly, then NaαCuβSγ converts into Na2S.35 And An et al. stated that Na+ insert into CuS to come into being intermedia NaCu2S2.36 Kalimuldina et al. and Wang et al. claimed that the different potential plateaus were closely associated with the involved phase transition and new phase formation.37,38 Though Li et al. had observed such potential plateau migration of Cu2S as anode in sodium ion batteries (SIBs), the detailed underlying reaction mechanism still remained unclear.39 Moreover, the origin of depolarization phenomenon during cycling require a comprehensively in-depth study, which may excavate some new clues to understand the reduction of potential plateau in CuxS anode and further improve the energy quality of SIBs. Herein, CuxS anode of sodium ion battery with high performance was prepared and investigated to elucidate the abonormal potential plateaus negative migration. An interesting Na+ storage mechanism evolution in the long-term cycling from intercalation-type reaction to conversion-type reaction was confirmed and intensively evidenced. This study may provide a new clue for other TMSs to reduce the operation voltage and improve the respective energy density.

2. Experimental 2.1. Materials preparation All chemicals used in experiment were purchased from Sinopharm and used directly without any further purification. The CuxS and C/N-CuxS composites were synthesized via wet-chemical reaction method according to our previous work.40 Typically, 1.01 g Cu(CH3COO)2 and 1.5878 g thioacetamide as sulfur source were separately dissolved in 30 ml anhydrous ethanol. And the solutions were mixed by adding Cu(CH3COO)2 into thioacetamide solution and 1.02 g L-tryptophan as the C/N

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source additive with vigorous agitation for 30 min. The mixture was then transferred into a 100 ml Teflon-lined stainless steel autoclave and remained at 180 ºC for 8 h with naturally cooled. The products were collected by centrifugation, washed with distilled water and absolute ethanol for several times, and dried at 80 ºC for 12 h in vacuum. Finally, the C/N-CuxS was obtained by annealing as-prepared composite at 500 ºC for 4 h in Ar. As comparison, the sample CuxS was prepared with the similar procedure as the preparation of C/N-CuxS without using 1.02 g L-tryptophan as the C/N additive. 2.2 Materials characterization Crystalline phase of the as prepared samples was determined by the powder Xray diffraction using Cu Kα radiation in the 2θ range of 20º–70º. The XRD data was refined by Rietveld method using PDXL software. The morphology and microstructure of the samples were observed by scanning electron microscope (SEM, HITACHI S-4800) and transmission electron microscopy (TEM, JEOL 2100F). The chemical states of elements were characterized by X-ray photoelectron spectra (XPS, Thermo Scientific ESCALAB 250Xi). 2.3 Electrochemical measurement The electrochemical measurements of these materials were carried out via CR2025-type coin cells assembled in an argon-filled glove box (Dellis company). The working electrodes were prepared by ball mill mixing 75 wt% active materials, 15 wt.% conductive materials (acetylene black) and 10 wt.% binder (CMC and SBR in ratio of 1:1) with deionized water as solvent. Subsequently, the slurry was spread on copper foil current collector by a coating machine. Then, the Cu foil with the slurry was dried at 120 ºC for 12 h in a vacuum oven. Then, the dried plate was cut into disk with a diameter of 1.5 cm. The mass loading of electrode is ~3 mgcm-2. Coin cells were fabricated with metallic sodium film as counter/reference electrodes and Whatman glass fiber filter as separator. And 1.0 M NaCF3SO3 in diethylene glycol dimethyl ether (DIGLYME) was used as the electrolyte. Ex-situ measurement was carried out with cells disassembled at different voltage. The obtained electrodes were washed several times with DME solve and dried overnight to evaporate the residual solvent. The galvanostatic charge/discharge tests was conducted on a Neware BTS610 battery test system in the voltage range of 0.4-2.6 V (vs. Na/Na+). The electrochemical impedance spectroscopy (EIS) plots were obtained using Zennium 3



IM6 electrochemical workstation with alternating-current amplitude of 5 mV in a frequency range of 10-1~105 Hz.

3. Results and discussion The powder XRD measurements were performed to investigate the phase structure and crystallinity of the prepared samples. The results indicated that the C/N additive would not influence the crystal structure (Figure S1). In addition, SEM images (Figure S2) present the similar morphology consisting of the nanoparticles of the two samples. As shown in Figure 1a, the diffraction peaks for samples could be assigned to Cu1.81S (PDF#41-0959) and Cu1.8S (PDF#23-0962) without other obvious impurity phase. And the refinement analysis of C/N-CuxS could help to confirm the composition of 68.47% Cu1.81S and 31.53% Cu1.8S. And the TEM image (Figure 1b) clearly displays the nanoparticles and nanorods in the as-prepared C/N-CuxS. The enlarged TEM image (inset in Figure 1b) clearly shows the nanoparticle of 50 nm and the nanorods of 40 nm. And the SAED pattern (inset in Figure 1b) exhibits several bright diffraction rings, which indicate the polycrystalline nature of typical samples. And all bright diffraction rings could be indexed to Cu1.8S and Cu1.81S phase. The XPS was carried out to further investigate the surface composition and element chemical state of C/N-CuxS anode of sodium ion battery. Two peaks at 932.4 eV and 952.4 eV could be detected in the Cu 2p high-resolution XPS spectrum (Figure 1c), which could be deconvoluted into four sub-peaks of Cu+ (2p3/2:932.4 eV, 2p1/2:952.4 eV)41,42 and Cu2+ (2p3/2:933.6 eV, 2p1/2: 953.7 eV)43,44. With regards to the S 2p high-resolution spectrum (Figure 2d), the binding energies centered at 161.5 and 162.6 eV are related to S 2p3/2 and S 2p1/2.45 And another peak at 164.5 eV correspond to C-S bond.46 As observed in Figure 2e, the C 1s peaks could be fitted and divided into five peaks. The peaks at 284.4, 285.1, 285.4, 286.3 and 288.7 eV could be indexed to the C-C, C-O, C=C, C=N and C-N bonds, respectively.47-49 From Figure 2f, the N 1s spectrum of C/N-CuxS reveals the existence pyridinic N, pyrrolic N, and amino N located at 401.5 eV, 399.9 eV, 398.8 eV, respectively.50 The electron diffraction X-ray spectroscopy (EDS) elemental mapping of the C/N-CuxS (Figure S3) demonstrates the uniform distribution of Cu, S, C and N components.

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Figure 1. (a) Rietveld refinement of XRD pattern, (b) TEM and SAED images of C/N-CuxS, Highresolution X-ray photoelectron spectra of the C/N-CuxS: (c) Cu 2p, (d) S 2p, (e) C 1s, (f) N 1s.

The average specific capacities of C/N-CuxS anode of sodium ion battery are 343.0, 307.9, 290.9, 284.5, 271.6, 224.0 mAhg-1 at 0.1, 0.2, 0.5, 1.0, 2.0 and 5.0 Ag-1, respectively. Besides, the CuxS electrode shows lower capacities, especially at high current density. The corresponding galvanostatic discharge−charge profiles were dispalyed in Figure S4. The C/N-CuxS electrode exhibits superior rate performance compared with CuxS, because the electron transportation could be improved by C/N co-doping. In Figure 2b, the cycle performance was verified at 1 Ag-1. The C/N-CuxS

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electrode maintain higher specific capacity of 274 mAhg-1 at 1 Ag-1 during cycle process compared with the CuxS electrode. Figure 2c, d display linear fit of average voltage vs. square root of the current intensity of C/N-CuxS and CuxS electrodes. The C/N-CuxS anode of sodium ion battery exhibits less ΔV at each current density and smaller absolute value of slope than those of the CuxS, indicating slighter polarization of the C/N-CuxS electrode material. The relative results have been apparently displayed in Table S1. The difference between charge and discharge average voltage of the C/NCuxS anode of sodium ion battery is relatively lower than that of CuxS. Thus, The C/NCuxS electrode delivers a competitive electrochemical performance in rate capability. Besides, the specific capacities of C/N-CuxS and CuxS composite electrodes would decrease at initial stage and keep stable in the following cycles (Figure 2b). The irreversible capacity loss results from the solid-electrolyte interphase (SEI) film stabilization on the surface of the samples, irreversible trapping of some sodium in the lattice of sulfide and sulfur dissolution into the electrolyte during the early sodiation process.51-53 The ex situ XPS analysis of the different cycled electrodes for S 2p spectrum exhibited in Figure 2e and Figure 2f. The peak at 168.6 eV in the S 2p spectrum corresponds to SOx, which might originate from the formation of SEI film with electrolyte salt (NaCF3SO3) after 15 cycles.54 Furthermore, the peak at 168.6 eV in S 2p spectrum also appears after 400 cycles and it maintains a similar area, which indicates the SEI film stability during the cycling process.

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Figure 2. (a) Rate capability of the C/N-CuxS and CuxS electrodes at different current rate from 0.1 to 5 Ag-1. (b) Cycling performance of the C/N-CuxS and CuxS electrodes at 1 Ag−1. Linear fit of the average voltage vs. the square root of the current intensity (c) C/N-CuxS and (d) CuxS electrode. Ex situ XPS of the different cycled electrodes for S 2p spectrum: (e) 15 cycles and (f) 400 cycles. (g) Specific capacity and coulombic efficiency with cycling at of 1 Ag-1.

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To demonstrate the long-term cyclic performance of C/N-CuxS electrode, the cell was cycled at 1 Ag-1 within 0.4−2.6 V after rate performance test. Figure 2g shows superior long-term cyclic performance of C/N-CuxS electrode at 1 Ag-1 and a reversible capacity of 266.93 mAhg-1 is maintained after 2000 cycles. After 2000 cycles, the C/N-CuxS electrode can deliver a high capacity retention of 93.53%, which proved ultrahigh cycle stability. Furthermore, the C/N-CuxS electrode maintains high coulombic efficiency maintains over 99% in the 2000 cycles. Such superior cycle stability at high current density indicates that the C/N-CuxS electrode possesses high reversible sodiation-desodiation process. A short comparison of the electrochemical performance with the previous reports in Table S2.

Figure 3. (a) galvanostatic discharge/charge profiles at a current density of 1 Ag−1 of the C/N-CuxS electrode in cycling process. (b) dQ/dV vs. V plot of the C/N-CuxS electrode at different cycles. Normalized capacity contribution ratio of redox peak at 1.53/1.92 V in discharge process (c) and 1.85/2.13 V in charge process (d).

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Figure 4. (a) Galvanostatic profile at 1 Ag-1 in 0.4–2.6 V. (b) Ex situ XRD patterns of C/N-CuxS electrode at 1.92 V (discharge), 1.53 V (discharge), 1.85 V (charge) and 2.13 V (charge) in the charge– discharge processes. (c-f) High-magnification TEM images and corresponding selected area electron diffraction (SAED) patterns observed from the electrode at 1.92 V (discharge), 1.53 V (discharge), 1.85 V (charge) and 2.13 V (charge).

Interestingly, the charge-discharge profiles corresponding to the galvanostatic cycle performance presents that two potential plateaus at 1.92 V and 2.13 V in the charge-discharge processes occurs obviously change with cycling proceed (Figure 3a). During discharge process, the plateau at 1.92 V becomes shorter while the plateau at 1.53 V keeps prolongation. Correspondingly, during charge process, a temporary potential plateau at 2.13 V in charge profiles would gradually disappeared and a fresh plateau at 1.85 V grows up gradually with increasing cycle number. Finally, the

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plateaus at 1.85 V and 1.53 V are observed with the plateaus at 1.92 V and 2.13 V vanished. The variability of the potential plateaus could also be realized in the differential capacity versus potential (dQ/dV) curves, as shown in Figure 3b. The oxidation peak gradually shifts from 2.13 V to 1.85 V with increasing cycle number. And the reduction peak at 1.92 V disappeared with a sharper reduction peak at 1.53 V during cycle processes. Furthermore, the change of redox peak intensity indicates that the electrochemical activity at 1.92 and 2.13 V become weak gradually with improved electrochemical activity at 1.85 and 1.53 V during the cycling process. Finally, the capacity contribution ratio of redox peak at 1.53/1.92 V in discharge process and 1.85/2.13 V in charge process was normalized in Figure 3b and 3c. With the increase of the cycle, the capacity contribution at 1.53 V gradually increased, while that at 1.92 V gradually decreased in discharge process. Meanwhile, the capacity contribution ratio at 1.85 V progressively enhanced, accompanied by that at 2.13 V constantly receded in charge process. And the capacity contribution evolution at different voltage plateaus infer thepotential plateaus negative migration with different reaction mechanism. In order to investigate the mechanism of potential plateaus change before and after, ex situ XRD experiment was carried out to study the phase transition of C/NCuxS anode at those plateaus (Figure 4a). The carbon black paper was used as the collector to avoid the interference of Cu-foil. The diffraction patterns are shown in Figure 4b. And the diffraction peak at around 26º could be identified as free carbon black paper. When the electrode was discharged to 1.92 V, the peaks at 23.76º, 36.31º and 39.93º appeared and could be assigned to (206), (235) and (143) crystal planes of Na4Cu2S3 (PDF#81-0343), respectively. And the peaks at 32.49º, 34.13º, 38.16º and 46.67º corresponded to (311), (401), (130) and (231) crystal planes of Na3Cu4S4 (PDF#71-1292), respectively. It is indicated that the series phase transformation triggered by the intercalation of Na+ into the Cu1.81S and Cu1.8S, which demonstrates that the reduction peak at 1.92 V in dQ/dV curves is identified to intercalation reaction and agree with the latest report.55 The latest research have reported that Na3Cu4S4 have been formed after Na+ insert into Cu1.8S in charge and discharge process.42 So, it could be confirmed that Na4Cu2S3 is the production of the intercalation of Na+ into Cu1.81S. And the peak intensity in the XRD patterns (Figure 4b) is weak but not strong due to the nanoparticles have been formed in reaction process.55 When it was discharged to 1.53 V, a new diffraction peak at 38.96º is derived from the (220) crystal planes of Na2S, a product of the Na+ conversion reaction. And a weak characteristic peak at 10


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43.47º that corresponds to the (111) plane can be assigned to the metallic Cu, demonstrating that the material has been reduced to metallic Cu after insertion of Na+. Figure 4c to f show HR-TEM images and SAED patterns of electrodes at 1.92 V (discharge), 1.53 V (discharge), 1.85 V (charge) and 2.13 V (charge). After discharging to 1.92 V (Figure 4c), the dominant (231) and (206) planes were observed with d-spacing of 1.95 Å and 3.74 Å, which are derived from Na3Cu4S4 and Na4Cu2S3 phase. It is also confirmed by the diffraction pattern in SAED image. Further discharging to 1.53 V (Figure 4d), the d-spacing of 2.08 Å and 2.31 Å is closely related to the (111) plane of metallic Cu (PDF#01-1242) and the (220) plane of Na2S phase (PDF#77-2149), which is consistent to the ex situ XRD. In corresponding SAED image, the (111) plane of metallic Cu and (200) plane of Na2S phase could be verified. The (200) plane of Na2S phase could not be detected in ex situ XRD. It could be because the diffraction peak of (200) plane at around 27º have been covered by the broadly diffraction peak of carbon paper. In desodiation process, the HR-TEM and corresponding SAED of electrodes charge at 1.85 V and 2.13 V are shown in Figure 4e-f. This invertible process of sodiation is agree with the ex situ XRD. Combining with ex situ XRD, HR-TEM image and SAED pattern all confirm that potential plateaus negative migration presence the intercalation reaction mechanism and conversion reaction mechanism. The potential plateaus negative migration suggests the possibility of dynamic cooperation between the intercalation and conversion reactions inside a Na-CuxS system with Na3Cu4S4+Na4Cu2S3, and Na2S+Cu phases co-existing. To be a short conclusion, the Na+ storage mechanism of the potential plateaus before migration could be ascribed to intercalation reactions with Na3Cu4S4 and Na4Cu2S3 products. When potential plateaus migration, the conversion reaction appeared with Na2S and metallic copper formed. And the conversion reaction dominated reaction progress with the further negative migration of potential plateau. As shown in Figure 5, the XPS of Cu element at discharged to 1.53 V and recharged to 1.85 V also found the exist of metallic Cu0, but the co-exist of Cu2+/Cu+ could be obtained at 1.92 V (discharge) and 2.13 V (charge) without the metallic Cu0. It is illustrated that the reduction peak centered at 1.92 V and 1.53 V could be assigned to the intercalation reaction and conversion reaction, respectively. This result is agreed with the results in Figure 4.

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Figure 5. Ex situ XPS patterns of C/N-CuxS electrode at two potential plateaus change before ((a) discharged to 1.92 V and (d) charged to 2.13 V) and after ((b) discharged to 1.53 V and (c) charged to 1.85 V) in the charge–discharge processes.

To interpret the potential platform shift kinetics, the activation energies (Ea) of the as-prepared C/N-CuxS anode of sodium ion battery for potential platform migration process were estimated by EIS. Figure 6a-d shows the EIS of the electrodes of C/NCuxS at different charge/discharge stages and temperatures. The ln(T/Rct) as a function of 1/T is shown in Figure 6a-d. And, the equivalent circuit model and calculated parameters is given in Figure S5. Based on the calculated process of intercalation of sodium ions in Supporting information, the exchange currents (i0) and the apparent activation energies (Ea) for the can be calculated. The activation energies of C/N-CuxS electrode at lower potential platform 1.53 V (discharge) and 1.85 V (Charge) are calculated to be 9.75 and 9.42 kJmol-1, respectively. In comparison, the activation energies of C/N-CuxS electrode at relative higher potential plateau 1.92 V (discharge) and 2.13 V (Charge) are calculated to be 11.39 and 11.60 kJmol-1, respectively. The C/N-CuxS electrode has lower activation energies at lower potential platform, which is indicated that the sodium ions are more facile intercalation and the material maintains higher electrochemical activity at lower potential. This enhanced kinetics could

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promote the migration of higher potential platforms to lower potential platforms gradually during charge/discharge process.

Figure 6. Electrochemical impedance spectra (EIS) and Arrhenius plots of ln(T/Rct) versus 1/T for the electrode of C/N-CuxS at (a) discharge 1.92 V, (b) discharge 1.53 V, (c) charge 2.13 V and (d) charge 1.85 V. (e) DNa+ calculated from GITT data for the C/N-CuxS electrode. (f) the DNa+ for the C/N-CuxS electrode at the position of potential plateaus migration.

To gain a better understanding of the the electrode platform shift kinetics with the diffusion of Na+ in C/N-CuxS anode of sodium ion battery, the Nyquist plots of C/NCuxS electrode at room temperature and different voltage position are shown in Figure S6a. In general, the low-frequency oblique line signifies the Warburg impedance (Zw), which is related to the sodium ions diffusion in the active material. Besides, Figure S6b exhibits the relationship between Z and the square root of the frequency (-1/2,  = 13



2ƒ) corresponding to the low-frequency sloping line portion. The lower slope of C/NCuxS electrode at lower potential plateaus indicates better sodium ion kinetics in the internal of electrode. Furthermore, the Na+ chemical diffusion coefficient of potential plateaus shift for C/N-CuxS electrode are determined via the galvanostatic intermittent titration technique (GITT).56 The GITT curve was shown in Figure S7. And Figure S8 exhibits a typical single-step GITT profile for the sample. Furthermore, Figure S9 reveal that the cell potential (E) is linearly proportional to τ1/2 for a single GITT curve during titration. Based on Equation and assumptions in supporting information, the values of DNa+ during the charge-discharge processes are calculated in Figure 6e. And Figure 6f gives the DNa+ vs. potential plots at four potential plateaus. As portrayed in Figure 6f, the value of DNa+ is calculated to be 3.32×10-13 cm2s-1 at 1.92 V (discharge) and then increase to 6.37×10-13 cm2s-1 with the potential plateau discharge at 1.53 V. Furthermore, the DNa+ is raised from 2.18×10-13 cm2s-1 to 4.41×10-13 cm2s-1 as the potential plateau shift from 2.13 V to 1.85 V. These results confirm that potential plateau negative migration is accompanied with the increasing of Na+ chemical diffusion coefficients in favor of the electrochemical performance, which exhibits a similar tendency and result to the diffusion of sodium ions in Nyquist plots. Combining the changed charge-discharge profiles, reaction mechanism and kinetics, it implies a dynamic cooperation between the intercalation and conversion reactions in a Na–CuxS system with Na3Cu4S4+Na4Cu2S3, and Na2S+Cu phases coexisting (Figure 7). The Na+ storage mechanism of the potential plateaus at 1.92/2.13 V and 1.53/1.85 V could be confirmed as intercalation and conversion reactions, respectively. Moreover, the potential plateaus at 1.92/2.13 V disappears gradually and the potential plateaus at 1.53/1.85 V grows up gradually during the cycling process which would be attributed to the Na+ storage mechanism of intercalation reaction gradually receded and conversion reaction gradual enhanced. Importantly, this dynamic competition is conducive to the development of the electrochemical properties of materials.

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Figure 7. Schematic illustration of the reaction mechanism evolution for CuxS anode.

4. Conclusions In summary, the C-N co-doped CuxS nanoparticles have been synthesized by onepot hydrothermal reaction method with further heat treatment in Ar. The C/N-CuxS electrode delivers a remarkable cycling stability and superior rate performance, when served as the anode material of rechargeable sodium ions batteries. A discharge capacity of 343.0 mAhg-1 at 0.1 Ag-1 have been achieved and a capacity retention of 93.53% at 1 Ag-1 after 2000 cycles. The potential plateau change was observed during the electrochemical cycling process. On the basis of the reaction mechanism and kinetics of Na+ intercalation/deintercalation process, the origin of potential plateau change has been carefully investigated. The research results indicated that the formation of Na3Cu4S4 and Na4Cu2S3 at 1.92/2.13 V could be attributed to the intercalation reactions. And the Na+ storage mechanism of conversion reaction is associated with the appearance of Na2S and metallic Cu at the potential plateaus 1.53/1.85 V, respectively. It implies the dynamic cooperation between the intercalation and conversion reactions inside a Na–CuxS system with Na3Cu4S4+Na4Cu2S3, and Na2S+Cu phases co-existing. Thus, the Na+ storage mechanism of intercalation reaction dominating the early migration process of potential plateaus transfers to the conversion reaction dominating reaction progress in the later period of potential

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plateau negative migration, which could illustrates that the potential plateaus at 1.92/2.13 V move towards a lower voltage during the cycling process. Finally, while this study focuses primarily on understanding the phenomenon of potential plateaus decay, it would be of interest to reveal the plateau migration mechanisms that providing an extraordinary comprehension distinction from other electrode material in SIBs.

Acknowledgements This work was supported by National Natural Science Foundation of China (Grant No.201506133), Distinguished Young Scholars of Sichuan University (2017SCU04A08). Research Foundation for the Postdoctoral Program of Sichuan University (No. 2017SCU12018, 2018SCU12045).

Supporting Information XRD patterns of the materials; SEM image of the as-prepared materials; EDS spectrum of the C/N-CuxS; the discharge-charge curves at different current densities; Equivalent circuit model of the studied system; EIS plots, equivalent circuit and GITT curves of C/N-CuxS electrode; the cell potential response with time of a discharge pulse for a single GITT titration; linear behavior of E vs. τ1/2 relationship in the discharge process; comparison of electrochemical performance with those of the previous reported works.

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Interpreting the Abnormal Charge-discharge Plateau Migration in

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