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Unraveling the Impact of Ether and Carbonate Electrolytes on the Solid-Electrolyte Interface and the Electrochemical Performances of ZnSe@C Core-Shell Composite as Anode of Lithium Ion Batteries Dejun Ma, Qiulan Zhu, Xintao Li, Hongcheng Gao, Xiufang Wang, Xiongwu Kang, and Yong Tian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21237 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 5, 2019
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ACS Applied Materials & Interfaces
Unraveling the Impact of Ether and Carbonate Electrolytes on the Solid-Electrolyte Interface and the Electrochemical Performances of ZnSe@C Core-Shell Composite as Anode of Lithium Ion Batteries Dejun Maa, Qiulan Zhua, Xintao Lia, Hongcheng Gaob, Xiufang Wanga, Xiongwu Kang*b and Yong Tian*a a
School of Pharmacy, Guangdong Pharmaceutical University, Guangzhou 510006, China Guangzhou Key Laboratory for Surface Chemistry of Energy Materials, New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou 510006, China E-mails:
[email protected];
[email protected] b
Key words: zinc selenide; solid electrolyte interface; lithium ion batteries; evolution dynamic; selenium dissolution; ex-situ XPS; ex-situ XRD
Abstract The recognition of solid electrolyte interface (SEI) between the electrode materials and electrolyte is limiting the selection of electrode materials, electrolytes and further the electrochemical performance of batteries. Herein, we report ZnSe@C core-shell nanocomposites derived from ZIF-8 as anode materials of lithium ion batteries (LIBs), the electrochemical performances and SEI films formed on ZnSe@C in both ether and carbonate electrolytes. It is found that ZnSe@C delivers a reversible capacity of 617.1 mAh·g–1 after 800 cycles at 1 A·g–1 in ether electrolyte, much higher than that in carbonate electrolyte. Both ex-situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) reveal that stable SEI films are formed on ZnSe@C in ether 1
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electrolyte while selenium is involved in the formation of SEI film and further dissolved into carbonate electrolyte due to the concurrent decomposition of electrolyte and insertion of Li+ into ZnSe, which differentiates between the cycling performances of ZnSe@C composite in ether and carbonate electrolyte.
Introduction Rechargeable lithium-ion batteries (LIBs) of high energy density, long cycling life and superior rate capability have been considered as a promising power source for portable electronic devices. Currently, the anode materials of commercial LIBs are mainly using carbonaceous materials, such as graphite and hard carbon, etc. due to their low cost and high stability. However, the low theoretical capacity of carbonaceous materials (372 mAh∙g–1)1-4 cannot meet the requirement of high capacity of electric vehicles and power grids. Additionally, the potentials of lithium insertion and deposition on carbonaceous materials are close to each other, leading to the formation of lithium dendrites and thus safety concerns. Thus, it is desirable to develop alternative anode materials for LIBs. Lately, it has been found that transition metal selenides (TMS) stored Li+ through reversible conversion reaction with the reaction product of Li2Se5, provide much larger theoretical specific capacities than carbonaceous electrode materials and are promising anode materials for high capacity LIBs. For example, CoSe6-7, MoSe8-9 and FeSe10-11 have shown great potentials as anode materials for both LIBs and sodium ion batteries 2
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(SIBs) and excellent electrochemical performances have been obtained. As an important member of transition metal selenide family, ZnSe has been widely studied as anode materials of LIBs due to its remarkable electrical conductivity and theoretical capacity (557 mAh·g–1)12-15. However, the large volume variation of TMS during the lithium insertion/extraction processes resulted in the pulverization of the anode materials, loose contact of active materials with the current collector and eventually the rapid fading of batteries capacity. Hybridization of TMS with carbonaceous materials derived from metal-organic frameworks (MOFs)16-17 not only accelerate ion and electron transport, but also mitigate the large volumetric and structural variation during cycling, thus improving the electrochemical performance of LIBs. For example, Qian et al. reported that NiS nanodots embedded by calcination of MOF-74(Ni) within porous graphitic carbon nanowires could deliver a reversible capacity about 300 mAh·g–1 after 100 cycles at a current density of 60 mA·g–1.18 Lou et al. reported that nanosized CoS2 bubble-like subunits electrodes synthesized by Co-based zeolitic imidazolate framework (ZIF-67) exhibited a highly stable capacity of 737 mAh·g–1 after 200 cycles at 1 A·g–1.19 More recently, hybrid ZnS/C, in which ultrafine ZnS nanorods rooted in the hollow carbon polyhedral, were prepared through Zn-based zeolitic imidazolate framework (ZIF-8) and exhibited a specific capacity of 840 mAh g-1 after 300 cycles at 600 mA·g–1.20 Besides the electrode materials, the formation of solid electrolyte interface (SEI) film is equally critical for the rational design and selection of anode materials and electrolyte for LIBs of high capacity and longevity. Currently, SEI films on carbonaceous 3
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anode materials in carbonate electrolyte have been well studied and found that it can remarkably improve the electrochemical performance of LIBs21 and Li-S batteries.22 Accordingly, we deeply believe that the formation of SEI shall be equally important for the electrochemical performance of TMS anode materials and recognition of the formation and evolution dynamics of SEI film on TMS anodes shall be critical for the rational design of anode and electrolyte materials for LIBs of high performance, which however remain unexplored.23-24 Herein, we developed ZnSe@C core-shell nanocomposites through selenization of ZIF-8 at 600 °C. With ether electrolyte, a reversible capacity as high as 617.1 mAh·g–1 and a coulombic efficiency of ≈100% are achieved after 800 cycles at current density of 1 A∙g–1 in a voltage range from 0.01 to 3.0 V. Ex-situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) at different charged and discharged sates revealed that the dissolution of selenium occurred through the formation of SEI films during charging/discharging process with carbonate electrolyte while stable SEI films was formed (Scheme 1), which is responsible for the excellent cycling stability of LIBs in ether electrolyte.
4
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ZnSe Carbon
SEI w/ Se In carbonate electrolyte
Cycling
(a) Selenization at 600℃ SEI w/o Se ZIF-8
ZnSe@C
Se dissolution
(b) Cycling
In ether electrolyte No Se dissolution
Scheme 1. Illustration of the formation of SEI films on ZnSe@C in (a) carbonate (top) and (b) ether electrolytes (bottom): selenium is involved in the formation of SEI film and further dissolved into carbonate electrolyte while no selenium is involved and stable SEI film is formed in ether electrolyte.
Results and discussion Fig. S1(a) shows the SEM images of the as-prepared ZIF-8, with a shape of rhombic dodecahedron, a smooth surface, and an average particle size of about 70 nm. The XRD pattern of ZIF-8 in the inset of Fig. S1(a)) signifies the successful preparation of phase-pure ZIF-8.25 The X-ray diffraction pattern of ZnSe@C obtained from carbonization and selenization of ZIF-8 is shown in the inset of Fig. 1(a), where the ZIF-8 pattern disappears and a new pattern of cubic zinc blende emerges (JCPDS 37-1463). From the diffraction peak of ZnSe (111), the lattice fringe of reflection (111) and the lattice constants of ZnSe nanoparticles were calculated to be d=3.27 Å and a=5.66 Å respectively, in agreement with earlier report12. The SEM image of ZnSe@C in Fig. 1(a) shows the carbonized ZIF-8 cubic nanoparticles fused together and formed interconnected carbon network, which are beneficial for expedited electron transport and 5
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ion diffusion, thus improving electrochemical performance of ZnSe@C anode materials.26 The diameter of ZnSe nanoparticles was determined to be around 50 nm from TEM images of ZnSe@C in Fig. 1(b), which is intimately wrapped by the porous carbon and demonstrated a core-shell structure. The thickness of the carbonaceous layers is determined to be around 20 nm and the lattice fringes of ZnSe (111) was measured to be 0.33 nm respectively, as shown in Fig. 1(c) 14, which is in agreement with that determined from XRD results in the inset in Fig. 1(a). The HAADF-STEM elemental mapping in Fig. 1(d) demonstrates that Zn and Se principally distribute evenly at the center of the ZnSe@C composite while C and N exist throughout the whole nanoparticles, suggesting the formation of a core-shell structure.
ZnSe@C
Intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
(111) (220) (311) (400) (331)
(422)
JCPDS 37-1463
20
40
60
80
2Theta (Deg.)
Fig. 1 (a) FE-SEM image; (b) TEM image; (c) HRTEM image; (d) HAADF-STEM elemental mapping for C, N, Zn and Se of ZnSe@C composite respectively. (The inset in panel (a) shows the XRD pattern ZnSe@C) The weight percentage of ZnSe and carbon contents in ZnSe@C composite was 6
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evaluated by TGA measurements in O2 atmosphere. As shown in Fig. S1(b), the weight loss of 47.1 % between 410 °C and 750 °C is ascribed to the burning of the carbon materials and oxidation of ZnSe. The XRD pattern of the TGA product in Fig. S1(c) is consistent with that of ZnO (JCPDS 36-1451), indicating complete transformation of ZnSe to ZnO and Se (g).6 According to the reaction of 2ZnSe (s) + O2 (g) = 2ZnO (s) + Se (g) and the weight percentage of ZnO of the TGA product, the weight percentage of ZnSe and carbon in ZnSe@C composite are calculated to be 93.8 % and 6.2 %, respectively, suggesting extremely high loading of active materials of ZnSe. Fig. S1(d) shows the nitrogen adsorption–desorption isotherm curves and pore size distribution of ZnSe@C
composites.
According
to
the
Brunauer-Emmett-Teller
(BET)
and
Barrett-Joyner-Halenda (BJH) method, the specific surface areas of ZnSe@C composite is determined to be 98.0 m2∙g–1 and the pore-size mainly distributed from 2 to 10 nm. Such large specific surface area and mesoporous structure of ZnSe@C composite may offer large interfaces for electrode-electrolyte contact and pathways for the diffusion of lithium ions, as well as additional space to mitigate the volume expansion and contraction of electrode materials during the ion insertion/desertion, thus improving the electrochemical performance of LIBs.27-29 ZnSe@C composite was further characterized by XPS measurements. From the full survey XPS spectra in Fig.2(a), C 1s, N 1s, O 1s, Zn 2p, and Se 3d are clearly observed.28, 30
In Fig.2(b), the high resolution XPS spectrum of C 1s is deconvoluted into three
components centered at 284.6±0.1, 285.6±0.1, and 287.5±0.2 eV, which are attributed 7
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to sp2 C, N-sp2 C and N-sp3 C bond,31 respectively. As shown in Fig.2(c), the high resolution XPS spectrum of N 1s is deconvoluted into three main contributions at 398.3 ±0.2 eV, 399.1±0.1 eV and 400.8±0.2 eV, which are assigned to pyridinic, pyrrolic and graphitic N respectively.32 The successful doping of nitrogen into carbon matrix not only increase the conductivity of carbon matrix, but also strengthen the coupling strength between TMS and carbon matrix.33 The binding energy of Zn 2p3/2 and Zn 2p1/2 is observed at 1021.4 eV and 1044.4 eV respectively (Fig.2(d)), while it is observed at 54.4 eV and 58.8 eV for Se 3d ( Fig.2(e)), ascribing to Se2– and Se-O bonds respectively.34 The charge states of both Zn and Se are consistent with that of ZnSe35. The electrochemical performance of ZnSe@C composite as anode materials of LIBs is tested with both ether and carbonate electrolytes. Fig. S2(a)-(b) show the representative discharge-charge voltage profiles of ZnSe@C with ether and carbonate electrolyte for the 1st, 2nd, 50th, 100th, and 150th cycles at the current density of 0.1 A∙g–1 within a potential range of 0.01-3.0 V versus Li/Li+. The discharge and charge capacities of LIBs with carbonate electrolyte in the first cycle are 869 and 660 mAh∙g–1, respectively, leading to an irreversible capacity loss of 24%. When using ether electrolyte, the discharge and charge capacities of LIBs in the first cycle are 926.8 and 658.6 mAh∙g–1, respectively, leading to an irreversible capacity loss of 29%. The composite electrode shows large irreversible capability loss in the initial cycle, which might be attributed to irreversible phase transition, decomposition of electrolyte and formation of SEI film.36-38 Fig. S2(c) and (d) show the cycling performance of ZnSe@C anode with ether and carbonate 8
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electrolytes at current density of 0.1 A∙g–1 respectively. ZnSe@C anode with ether electrolyte exhibit quite stable cycling performance and deliver a specific capacity of 712.1 mAh∙g–1 at 400th cycle. However, the capacity of ZnSe@C anode with carbonate electrolyte goes up and down with cycling numbers, reaches the maximum of 770
0
(c)
Zn 2p 1/2
Zn 2p 3/2
O 1s
Zn LMM1 Zn LMM
N 1s
Zn 3p Se LMM1
(a)
Se LMM2 C 1s
mAh∙g–1 at 100th cycle and then decreases to 135 mAh∙g–1 at 400th cycle.
Se 3d
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Raw Intensity Peak Sum Background sp2C
(b)
N-sp2C N-sp3C
300
600 Binding energy (eV) Raw Intensity Peak Sum Backbround Pyridinic N Pyrrolic N Graphitic N
393 396 399 402 405
900
282 285 288 291 294
Binding energy (eV) Raw Intensity Peak Sum Backbround Zn 2p 3/2 Zn 2p 1/2
(d)
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Raw Intensity Peak Sum Backbround Se 3d Se-C or Se-O
54
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Binding energy (eV) Binding energy (eV) Binding energy (eV) Fig. 2 XPS spectra of ZnSe@C composite: (a) full survey and high-resolution spectrum of (b) C 1s; (c) N 1s; (d) Zn 2p; (e) Se 3d. Fig. 3(a) and (b) display a series of discharge–charge curves of ZnSe@C anode materials with ether and carbonate electrolytes at various current densities from 0.1 to 10 A∙g–1 respectively, and much better-kept potential platforms and higher capacities of LIBs are observed with ether electrolyte than that with carbonate electrolyte. The rate 9
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performance of ZnSe@C anode with carbonate and ether electrolytes are further shown in Fig.3(c). ZnSe@C anode materials combined with ether electrolyte deliver a specific capacity of 678.1, 622.2, 532.8, 454.5, 385.3, 302.9 and 250.5 mAh∙g–1 at 0.1, 0.2, 0.5, 1, 2, 5 and 10 A∙g–1. When the current density is reset to 0.1 A∙g–1, ZnSe@C can also deliver a specific capacity of 660 mAh∙g–1, recovering 97% of its initial capacity. Instead, when combined with carbonate electrolyte, ZnSe@C delivers a specific capacity of 620, 444, 328, 273, 235, 165 and 119 mAh∙g–1 at current rate of 0.1, 0.2, 0.5, 1, 2, 5 and 10 A∙g–1 respectively, much lower than that achieved by ether electrolyte. In addition, only 80% of its initial capacity was recovered when the current is set back to 0.1 A∙g–1. The discussion above suggests that ZnSe@C demonstrates much better rate performance in
2.5
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Fig. 3 Charge and discharge curves of ZnSe@C anodes with (a) carbonate and (b) ether 10
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Coulombic Efficiency (%)
120
Capacity (mAh/g)
(c) 1200
Potential (V)
(b) 3.0
Potential (V)
(a) 3.0
Coulombic Efficiency (%)
ether electrolyte than that in carbonate electrolyte.
Capacity (mAh/g)
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electrolyte at current densities from 0.1 A∙g–1 to 10 A∙g–1; (c) rate performance and (d) cyclic performance of ZnSe@C anodes with ether and carbonate electrolyte at 1 A∙g–1 and 2 A∙g–1. Fig. 3(d) displays the cycling performances and coulombic efficiency of ZnSe@C in ether and carbonate electrolytes at current density of 1 A∙g–1 and 2 A∙g–1 respectively. Remarkably, ZnSe@C electrode in ether electrolyte demonstrates exceptionally stable cycling performance due to the formation of the stable SEI film, delivering a consistent capacity of 617.1 and 472.3 mAh∙g–1 from the 70th to the 800th cycle at current density of 1 and 2 A∙g–1 respectively. In sharp contrast, the capacity of ZnSe@C electrode in carbonate electrolyte increases gradually from the 40th cycle and reached a value as high as 704.9 mAh∙g–1 at 300th cycle and then rapidly fade to 344.6 mAh∙g–1 at 800th cycle at current density of 1 A∙g–1. The increasing capacity in carbonate electrolyte in the early stage of the cycling is ascribed to the synergy of reversible formation and dissolution of polymeric and gel-like SEI film and the storage of the lithium ions through such interface,39-41 while the fast capacity decay might be due to the structural collapse, dissolution of active materials through the gel-like SEI film and electrolytes consumption during discharge/charge cycles. These factors will be further analyzed below by XPS, XRD and Electrochemical impedance spectroscopy (EIS) and discussed in details accordingly. It should be noted that the specific capacity of ZnSe@C with ether electrolyte delivers slightly lower specific capacity (617 mAh∙g–1) at 800th cycle with current density of 1 A∙g–1 than that at 400th cycle with current density of 0.1 A∙g–1 (712 mAh∙g–1), further suggesting the excellent rate and cycling performance of ZnSe@C. 11
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Surprisingly, ZnSe@C anode with carbonate electrolyte delivers much better cycling performance and higher capacity at current density of 1 A∙g–1 than 0.1 A∙g–1, indicating that it might be suitable for application in fast charge/discharge. Fig. S5 shows the cycling performance of carbon matrix derived from ZIF, which delivers only a capacity of 305 mAh∙g–1 after 100 cycles at 1A∙g–1. By considering the 6.2wt% of carbon matrix in ZnSe@C, the capacitance contribution from carbon matrix in ZnSe@C is completely negligible. The electrochemical performances of various TMS as anode materials of LIBs reported in the literature is compared with this work, as shown in Table S1. The cycling performance of ZnSe@C with ether electrolyte at current density of 0.1 A∙g–1 and 1 A∙g–1 surmount most of the anode materials reported in the literature, as shown Fig. S3. This suggests that ZnSe@C anode and ether electrolyte a good combination for LIBs of high performance. To investigate the mechanism of energy storage and the charge and discharge products of ZnSe@C, CV scans in a potential window from 0.01 to 3.0 V at a scan rate of 0.2 mV·s–1 and ex-situ XRD measurements at different charge and discharge state of ZnSe@C with ether and carbonate electrolytes are conducted respectively (Fig. 4). Fig. 4(a) shows the first cycle of CV scan of ZnSe@C with carbonate electrolyte, a sharp cathodic redox peak is observed at 0.4 V, which are derived from combined electrolyte reduction and conversion of ZnSe to LiZn and Li2Se (eq 1). In the subsequent CV scans, two cathodic peaks are observed at 0.72 V and 0.45 V, which might be attributed to conversion reaction of ZnSe to Zn and Li2Se (eq 2) and alloying reaction of Zn to LiZn 12
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(eq 3), respectively. The two anodic peaks at 0.55 V and 0.69 V might correspond to the dealloying process of LiZn to metallic Zn (eq 5).12 When further charged to 1.4 V, the anodic peak might be associated with the conversion reaction of Zn and Li2Se to ZnSe (eq 6).12-13 The anodic peak appeared near 2.5 V may be related to oxidation reaction of carbon matrix, as testified by the CV scans of bare ZnSe (Fig. S4(a)) and carbonized ZIF-8 matrix with Zn etched off (Fig. S4(b)). The overall charging reaction of ZnSe is described by eq 4. The CV plots of ZnSe@C are almost overlapped during the subsequent scans, indicating the reversible electrochemical reactions and high stability of the anode materials. Discharging process: ZnSe + 3Li+ + 3e– → LiZn + Li2Se ZnSe + 2Li+ + 2e– → Zn + Li2Se
(eq1) (eq 2)
Zn + Li+ + e– → LiZn
(eq 3)
LiZn + Li2Se → ZnSe + 3Li+ + 3e–
(eq4)
Charging process:
LiZn → Zn + Li+ + e– Zn + Li2Se → ZnSe + 2Li+ + 2e–
13
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(eq5) (eq6)
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1.0
Current (mA)
0.5
Current (mA)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
(a)
ZnSe ★
■
(b)
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Li2Se ▲ copper foil
● film
◆
Zn
▲ ●
0.0
●
-0.5 -1.0
1st
2nd
-1.5 1.0
4th
5th
0.5
charge 3.0V charge 1.7V charge 1.3V charge 0.7V discharge 0.01V discharge 0.5V ■ ■ primeval film and copper ■
★ ★ ★ ★ ★ ★ ★ ■
3rd
(c)
▲
■ ■ ◆
■
(d)
◆
ZnSe ★
Li2Se ▲ copper foil
● film
◆
Zn
▲
0.0
●●
-0.5 -1.0
★ ★
1st
2nd
4th
5th
★
3rd
★ ■
charge 3.0V charge 2.0V charge 1.5V charge 1.0V discharge 0.01V discharge 0.5V discharge 1.0V ■ ■ primeval film and copper ▲
■ ■ ◆ ◆
■ ■
-1.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 20 30 40 50 60 0 1 2 3 2Theta (Deg.) Votage (V) Potential (V) Fig. 4 The initial five successive CV curves in the potential range of 0.01 to 3 V at a scan rate of 0.2 mV∙s–1 with (a) carbonate and (c) ether electrolytes and ex-situ XRD patterns at different charge−discharge voltages at 50 mA·g–1 of ZnSe@C with (b) carbonate and (d) ether electrolyte.
Such charge and discharge reactions stated above are further corroborated by ex-situ XRD in Fig. 4 (b), where the discharged and charged voltages are selected according to the CV measurements in Fig. 4(a). Before discharging, three diffraction peaks of ZnSe are observed at 27.3°, 45.4° and 53.7° (black curve). When discharged to 0.5 V, the potential right before the voltage of the first cathodic peak, the characteristic peak of Li2Se was observed at 26.4° (highlighted in red square), suggesting the conversion reaction of ZnSe to Li2Se. Further discharge to 0.01 V results in the thorough conversion of ZnSe to Li2Se, which slightly shifts diffraction peaks towards lower angle. When charged back to 0.7 V, the potential slightly higher than that for the conversion of LiZn to Zn, the diffraction pattern of Zn appeared at 36.3°, and 39.0°. Further charging to 1.3 V 14
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results in the disappearance of the diffraction pattern of Zn and reduced peak intensity of Li2Se, possibly due to the partial conversion reactions of Li2Se and Zn to ZnSe. When further being charged to 1.7 V, the potential after finishing the conversion reaction of Li2Se and Zn to ZnSe, the diffraction peaks of Li2Se are completely gone and diffraction peaks of ZnSe emerge, suggesting the successful conversion of Li2Se to ZnSe. When fully charged to 3 V, no new diffraction peak is observed, suggesting that the conversion reaction of Li2Se to ZnSe has been finished before the charging potential of 1.7 V and the anodic peak at 2.5 V in the CV may not bring new phase transformation, but just the redox reaction of carbon matrix. When using ether electrolyte, the potential of the first cathodic peak of CV scan (0.52 V) is slightly higher than that with carbonate electrolyte (0.40 V) (Fig. 4 (c)) due to the higher polarization in the latter electrolyte. Most importantly, well-defined ZnSe peaks are observed when discharged to 1.0 and 0.5 V (Fig. 4(d)), suggesting that no conversion of ZnSe to Li2Se occurs. In sharp contrast to that in carbonate electrolyte, Li2Se diffraction peaks emerge and conversion of ZnSe to Li2Se occurs at this potential. Until fully discharged to 0.01 V, diffraction feature of Li2Se start to emerge, which is highlighted in the red square in Fig.4 (d). This indicates that the electrolyte decomposition reaction and Li insertion to ZnSe in ether electrolyte may occur independently. The rest of the diffraction patterns of ZnSe@C electrodes at various charged and discharged states are quite consistent with that in carbonate electrolyte. These XRD results strongly corroborate the charge and discharge reaction processes 15
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derived from CV measurements.
(a)
1021.1 eV
charge 3.0 V
(b)
165.8 eV
160.0 eV
166.0 eV
159.9 eV
165.9 eV
159.5 eV
165.5 eV
discharge 0.01 V
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discharge 0.01 V 166.8 eV
discharge 0.5 V
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discharge 0.5 V 166.2 eV
discharge 1.0 V
160.4 eV
discharge 1.0 V 166.0 eV
primeval
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166.0 eV
charge 2.0 V
charge 2.0 V
1020.8 eV
1043.8 eV
1020.7 eV
1043.7 eV
1020.4 eV
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1020.4 eV
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1043.9 eV
1021.4 eV
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1021.4 eV
1044.4 eV
charge 1.5 V
charge 1.5 V
1020
1030
charge 3.0 V
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1044.1 eV
charge 1.0 V
charge 1.0 V
1040
1050
155
Binding energy (eV)
160
165
primeval
170
Binding energy (eV)
Fig. 5 Ex-situ XPS high resolution spectra of ZnSe@C at different charge−discharge states in ether electrolyte at a current density of 50 mA∙g−1: (a) Zn 2p and (b) Se 3p.
To study the chemical states of the compositions of ZnSe@C electrode at different discharged and charged states by ex-situ XPS, the SEI films formed on the ZnSe@C composite surface are removed by Ar ion sputtering. Fig. S6 shows the XPS full surveys of ZnSe@C charged and discharged at different voltages, from which the main components of N, Se, Zn, Li, F and O are clearly observed. Fig.5 (a) shows the high resolution XPS spectra of Zn 2p and Se 3p at different discharged and charged states. Before discharging, XPS spectra of Zn and Se demonstrate the generic feature of ZnSe, with the binding energy of Zn 2p3/2 and Zn 2p1/2 at 1021.4 and 1044.4 eV and Se 3p3/2 and Se 3p1/2 at 160.4 and 166.0 eV respectively. When discharged to 1.0 V, no shift of 16
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binding energy of Zn and Se is observed, suggesting that no conversion reaction to ZnSe happens when discharged to potentials higher than 1.0 V. Further discharging to 0.5 V and 0.01 V leads to the consistent decrease of the binding energies of Zn 2p to 1020.4 eV and 1043.5 eV and Se 3p of ZnSe@C to 159.9 and 166.8 eV respectively, signifying the conversion of ZnSe to Zn, LiZn and Li2Se. When further charged back to 1.0, 1.5, 2.0 and 3.0 V, Zn 2p are approximately returned to its original charge states, indicating the conversion of LiZn and Li2Se to ZnSe, in agreement with that observed in operando XRD measurements. However, the binding energy of Se still is more negative than that of the original ZnSe, possibly suggesting partial formation of Se and LiSex. To further unravel the difference of the electrochemical behaviors of the ZnSe@C in ether and carbonate electrolytes, XPS spectra of SEI film on ZnSe@C at different charged and discharged states are investigated, as shown in Fig.6 (a) and (b). Surprisingly, no feature of Zn, Se and N of the generic features of ZnSe and N doped carbon matrix are observed, suggesting the formation of SEI film on ZnSe@C surface due to electrochemical decomposition of electrolyte and reactions with electrode materials. The most prominent feature of the XPS spectra of SEI film for ZnSe@C with ether and carbonate electrolyte is that clear features of Se-LMM is observed on SEI film in carbonate electrolyte in Fig. 6 (b) when charged back to 1.3 and 1.5 eV, which start to become weaker when further charged to 1.7 V and eventually vanish at 3.0 V. The evolution dynamics of Se in SEI film in carbonate electrolyte might suggest that Se was involved in the formation of SEI film. It is highly possible that when charged to higher 17
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voltage, Li2Se might be oxidized to Se, further converted to polyselenides, dissolved into electrolyte and thus not observed. The dissolution of polyselenides is further supported by the observation of the yellow color of separator film in carbonate electrolyte after 150 cycling (Fig. S6(a)), which might come from the deposition of polyselenides on the separator film.42-43 Thus, ex-situ XPS analysis at various charged states indicates the reversible formation/dissolution of SEI film and the involvement of Se in the SEI film may also contribute to the lithium ion storage, thus resulting in the increasing capacity of LIBs in carbonate electrolyte in the early cycles. Continuous dissolution of active
charge 2.0 V
Cl 2p
Se LMM1
charge 3.0 V
Se 3p
(b)
Se LMM2
(a)
S 2s
materials Se into electrolyte may account for the fast capacity decay after the 300th cycle.
S 2p
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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charge 1.5 V
charge 3.0 V charge 1.7 V charge 1.5 V
charge 1.0 V charge 1.3 V
discharge 0.01 V
charge 0.7 V
discharge 0.5 V discharge 1.0 V
discharge 0.01 V discharge 0.5 V
150 180 210 240 270
150 180 210 240 270
Binding energy (eV) Binding energy (eV) Fig. 6 Ex-situ XPS spectra of SEI film formed on ZnSe@C in (a) ether and (b) carbonate electrolyte at different charge−discharge states at a current density of 50 mA∙g−1
In contrast to that in carbonate electrolyte, Se is not observed in the full survey spectra of ZnSe@C in ether electrolyte at all voltages and no selenium deposition on the separator film occurs after 150 cycles (Fig. S7). This suggests that no selenium is 18
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involved in the formation of SEI film and thus no dissolution of selenium into electrolyte occurs during charging and discharging process. Such differences of SEI films on ZnSe@C and Se involvement in both carbonate and ether electrolyte are illustrated in Scheme. 1. It is highly possible that the simultaneous decomposition of carbonate electrolyte and insertion of Li into ZnSe result in the involvement of Se in SEI film at low discharging potentials and further dissolution of Se at high charging potentials (Scheme. 1(a)), while the decomposition of ether electrolyte and insertion of Li into ZnSe occurs independently and thus no Se is involved in the formation of SEI film, which further result in stable SEI film and might be responsible for the superior cycling performance of ZnSe@C in ether electrolyte. In addition, the peaks of S 2p are observed in SEI film in ether electrolyte at all charged and discharged states (Fig.6(a)), which are mainly attributed to -SO2CF3 (169.0 eV) and Li2S (160.3 eV),44 as shown in the high resolution XPS spectra of S 2s in Fig. S8, which might be from LiTSFI decomposition in ether electrolyte. The SEM images of the ZnSe@C electrodes with carbonate and ether electrolytes after 150 cycles at 0.5 A∙g–1 are shown in Fig. S9, respectively, where much rougher surface of ZnSe@C is observed in carbonate electrolyte than in ether electrolyte, suggesting that the SEI layer in ether electrolyte is more stable than in carbonate electrolyte. Fig. S10 shows the high resolution XPS spectra of C 1s, F 1s and O 1s of SEI film formed on ZnSe@C electrode with ether and carbonate electrolytes when discharged to 0.01 V at a current density of 50 mA∙g−1. In Fig. S10 (a) and (b), the C 1s spectra of the 19
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SEI film formed in the two electrolytes are observed at 290.0 and 289.7 eV, which are assigned to Li2CO3 and ROCO2Li,45 respectively, due to the decomposition of the electrolytes. The other C 1s peaks located at ≈ 284.0-287.0 eV are attributed to C-C (graphite, 284.4 eV) and C-O or amorphous carbon (285.0 eV).22 It is noteworthy that the intensities of the C 1s peaks of SEI film formed in ether electrolyte are much weaker than those formed in carbonate electrolyte, indicating that the decomposition of the electrolyte solvents and salt anions in ether electrolyte are much less than those in carbonate electrolyte.46
(b)
200
120
fresh ether carbonate
150
100
Z'' (Ω)
80
100 50
10
150 cycles ether carbonate
60
Z'' (Ω)
(a)
Z'' (Ω)
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8 6 4 2 0 10
15
20 Z' (Ω)
25
30
40 20
0 0
0 100 150 200 0 20 40 60 80 100 120 Z' (Ω) Z' (Ω) Fig. 7 EIS spectra of ZnSe@C in carbonate and ether electrolyte, before (a) and after150 cycles (b) at 0.5 A∙g–1. The open circles represent the raw data and the solid lines are the fitting plots to the Randles circuits. The inset in panel (b) shows the zoom-in EIS spectra in ether electrolyte.
50
In Fig. S10 (c) and (d), F 1s of SEI film formed in ether and carbonate electrolytes observed at 684.5 eV is attributed to LiF, which is another main component of SEI film.22,
47
Interestingly, two species of F is observed for SEI film formed in ether
electrolyte. The peak at 684.5 eV is equally assigned to LiF, while that at 688.5 eV is 20
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ascribed to the C-F bond of the -CF3 group in the TFSI– anion.46-48 In Fig. S10 (e), the O 1s peak of SEI film formed in ether electrolyte is deconvoluted into two contributions: the one at 531.0 eV belongs to C-O and the other one at 532.0 eV is assigned to Li2CO3. In contrast, as shown in Fig. S10 (f), an additional O 1s peaks at 528.0 eV and 530.0 eV are observed and attributed to Li2O and LiOH for the SEI film formed in carbonate electrolyte.48-49 The disappearance of Li2O during the charging process (Fig. S11) suggests the large variation of the SEI film formed in carbonate electrolyte, which may result in selenium draining through the SEI layer. EIS spectra of ZnSe@C electrodes in both ether and carbonate electrolytes before and after 150 cycles is investigated at fully charged state (Fig. 7), further fitted to Randles equivalent circuits, and the fitting parameters are shown in Table. S2. It is observed that the electrolyte resistance in both electrolytes is quite the same, but the interfacial charge transfer resistance (Rct) derived from the semi-circle is quite different for the two electrolytes, which in carbonate electrolyte (146.00 Ω) is about twice that in ether electrolyte (67.50 Ω). This suggests much slower Faradic process at ZnSe@C electrodes surface in the former electrolyte than the latter.50-52 After 150 cycles, much more enhancement of the electrolyte resistance is observed in carbonate electrolyte than that in ether electrolyte, suggesting much faster electrolyte consumption rate in the former electrolyte, which may contribute to the fast capacity decay of LIBs after the 300th cycle in carbonate electrolyte. Besides, an additional semi-circle ascribing to the formation of SEI film and film resistance (Rf) is observed for both electrolytes. Again, 21
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both Rf and Rct in ether electrolyte are much lower than that in carbonate electrolyte, indicating faster lithium ion diffusion in the SEI film and Faradic process at the electrode/electrolyte interface and superior cycling performance in the former electrolyte.53 Conclusions In summary, we have developed a novel and efficient MOFs template-engaged formation of 3D hybrid composites constructed by ZnSe@C as advanced anode materials for LIBs. The rechargeable LIBs with ZnSe@C as the cathode and ether-based electrolyte show much better electrochemical performance than those with carbonate-based electrolyte. LIBs with ether-based electrolyte delivered capacity of 617.1 mAh∙g–1 and 472.8 mAh∙g–1 after cycling 800 times at 1 A∙g–1 and 2 A∙g–1, respectively. The operando XRD and XPS revealed the energy storage mechanism of ZnSe@C composite, which involved the conversion reaction of ZnSe to Li2S and ZnLi. The operando XPS of the SEI films on ZnSe@C composite showed the dissolution of selenium in carbonate-based electrolyte while much stable SEI film in ether-based electrolyte lead to the long cycling performance of ZnSe@C composite. This work not only offered a novel strategy of preparing anode materials, but also studied the evolution of ZnSe and impact of the electrolyte on the SEI film formation, which might offer insight for the rational design of anode and electrolyte materials for LIBs of high performance. Notes The authors declare no competing interest. 22
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Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 51602106 and No. 21373061), the Fundamental Research Funds for Central Universities (SCUT Grant No. 2017MS066 and 2017MS006). Prof. Z. L. would like to thank the Guangdong
Innovative
and
Entrepreneurial
Research
Team
Program
(No.
2016ZT06N569). Supporting Information Available: FE-SEM and XRD patterns of ZIF-8; TGA of ZnSe@C and XRD pattern of TGA product ZnO in O2 atmosphere; N2 adsorption-desorption profiles of ZnSe@C; electrochemical performance of ZnSe@C as with ether and carbonate electrolyte at a current density of 0.1 A∙g–1; literature survey of electrochemical properties of various typical transition metal selenides as anode materials for LIBs in the literatures; CV curves of bare ZnSe and carbonized ZIF-8 matrix with Zn etched as anode materials with carbonate electrolyte; operando XPS full survey spectra of ZnSe@C at current density of 50 mA∙g−1; operando XPS S 2s and O 1s spectra of SEI film in ether electrolyte; FE-SEM images of ZnSe@C and digital images of separators with carbonate and ether electrolyte after 150 cycles at 0.5 A∙g–1.
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TOC SEI w/ Se Cycling Carbon
(a)
ZnSe
In carbonate Se dissolution electrolyte
ZnSe@C
(b)
SEI w/o Se Cycling
In ether electrolyte
No Se dissolution
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