Control of SEI Formation for Stable Potassium-Ion Battery Anode by Bi

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Control of SEI Formation for Stable Potassium-Ion Battery Anode by Bi-MOF-Derived Nanocomposites Sailan Su, Qian Liu, Jue Wang, Ling Fan, Ruifang Ma, Suhua Chen, Xu Han, and Bingan Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06379 • Publication Date (Web): 29 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019

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ACS Applied Materials & Interfaces

Control of SEI Formation for Stable Potassium-Ion Battery Anode by Bi-MOF-Derived Nanocomposites Sailan Sua#, Qian Liua,b#, Jue Wanga, Ling Fana, Ruifang Maa, Suhua Chena, Xu Hanb and Bingan Lua* aSchool bState

of Physics and Electronics, Hunan University, Changsha 410082, China.

Key Laboratory of Advanced Design and Manufacturing for Vehicle Body,

College of Mechanical and Vehicle Engineering, Hunan University, Changsha 410082, China. #

These authors contributed equally to this work.

ABSTRACT: Bismuth (Bi) based electrodes are highly attractive for potassium-ion batteries (PIBs) while suffering from a short cycle life due to the larger diameter of K ion leading unstable solid electrolyte interface (SEI) films during continuous potassiation/depotassiation. Herein, we developed a novel ultra-thin carbon film@carbon nanorods@ Bi nanoparticles (UCF@CNs@BiNs) materials for the long cycle life anode of PIBs. Bi nanoparticles are uniformly distributed in the carbon nanorods, which not only provides a high-speed channel for ion transport and also accommodates the volume change of Bi nanoparticles during continuous potassiation/depotassiation processes. The UCF@CNs matrix can direct most SEI films formation on the surface of the carbon film, not on the surface of individual Bi nanoparticles, and avoiding the fracture of the matrix. Benefiting from the unique structure, the UCF@CNs@BiNs anodes exhibit an outstanding capacity of ~425 mAh g−1 at 100 mA g−1 and a capacity decay of 0.038 % per cycle over 600 cycles. Even at a higher current density of 1000 mA g−1, there is a capacity decay as low as 0.036 % per cycle during 700 cycles. Meanwhile, this work provides a new way of utilizing the MOF structure and reveals a highly promising PIBs anode. KEYWORDS: MOF, ultra-thin, carbon nanorods, Bi nanoparticles, potassium ion batteries 1. INTRODUCTION With the continuous development of the economy, the energy problem is becoming more and more serious, which needs to be addressed urgently.1-2 Even though lithium ion batteries (LIBs) possess the high energy and power density, the high cost and limited lithium resources restrict the further applications for large scale electric energy storage systems.3-6 An alternative is to pursue for other metal ion based batteries.7-10 Therefore, sodium ion batteries (SIBs) and potassium ion batteries (PIBs) have been extensively studied owing to their earth abundance, low cost and relatively low redox potential.11-15 Recently, enormous efforts have been put into SIBs and a great progress has been achieved.16-18 However, PIBs have more advantages, especially considering that the low redox potential of K+/K which is close to that 1 ACS Paragon Plus Environment

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of Li+/Li (-2.93 V vs.-3.04 V) and the measured mobility of K in the electrolyte is higher than that of Li (K, unlike Na, can intercalate into graphite.)7, 19-20 All of these merits of potassium make PIBs are considered as one of the most desirable alternatives for large scale electric energy storage systems.21-22 During the past years, the research on electrode materials of PIBs was extensive.23-29 Up until now, many electrode materials have been explored in PIBs, such as carbonaceous materials, alloying materials, intercalation anodes, organic anodes, et al.30-35 Some of alloy anodes have shown promising initial performance in storing K, such as alloying anode Tin (Sn), phosphorus (P), antimony (Sb), and bismuth (Bi), which have aroused tremendous interest owing to their appropriate potential and high theoretical specific capacities.36-41 Among them, Bi, a particularly stable pnictogen element which is earth abundant and non-toxic with a large lattice fringe, is a promising anode material for PIBs, delivering a relatively high theoretical capacity of 385 mAh g−1 and a low charge voltage.42-46 Recently, Bi has been reported as a promising anode material for PIBs, nevertheless Bi-based materials display the inferior rate capacity and cyclic stability, which are usually due to the low conductivity and the collapse of the structure stemming from the huge volume change (huge volume change leading repeated SEI films formation).42-45, 47 Herein, we report the novel ultra-thin carbon film@carbon nanorods@ Bi nanoparticles (denoted as UCF@CNs@BiNs) composites by directly annealing the Bi-based metal organic framework (Bi-MOF) as anodes for PIBs. The enhanced electrochemical performance of UCF@CNs@BiNs composites is owing to the UCF@CNs matrix, which can limit most solid electrolyte interface (SEI) films to the surface of carbon film, thus prevent fracture of the matrix. The carbon nanorods of MOF-derived UCF@CNs@BiNs can effectively buffer the volume change caused by alloying during the potassiation and depotassiation processes, thus extending their cycle life. Also the homogeneous distribution of Bi nanoparticle leads to the short ion diffusion and reduces the strain formation during discharge and charge process. As a result, the UCF@CNs@BiNs anode exhibited a superior cycle stability and high reversible capacity, realizing a high capacity over ~425 mAh g−1 at a current density 100 mA g−1 and a capacity decay of 0.038 % per cycle after 600 cycles. Furthermore, a steady long cycle over 700 cycles with a capacity decay of 0.036 % per cycle is obtained at a higher current density of 1000 mA g−1.This stable MOF derived UCF@CNs@BiNs material provided a new method for the application of MOF structure, which is critical to understand the electrochemical process of Bi-based alloying reactions. 2. EXPERIMENTAL SECTIONS 2 ACS Paragon Plus Environment

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2.1 Synthesis of UCF@CNs@BiNs The detail of synthetic procedure for the UCF@CNs@BiNs is listed as follows: The preparation is schematically illustrated in Figure 1. the Bi-MOF was prepared as a precursor following the methods reported in the literature. The synthesis was carried out under solvothermal conditions in a 100 ml Teflon lined steel reactor. In a typical synthesis, 3.659 mmol H3BTC (≥ 98 %, Alfa Aesar Co., Ltd) and 0.309 mmol Bi(NO3)3·5H2O (≥ 99 %, Sinopharm Chemical Reagent Co., Ltd) were dissolved in 60 mL methanol at room temperature. Upon continuously stirring, the transparent mixtures were transferred into the reactor after the powder was dissolved. Then the reactor was heated to 120 oC for 24 h. The obtained white powder was washed with methanol for several times, and thereafter separated by centrifugation. The final products were acquired after drying in an oven (60 oC, 3 h). Specifically, the composite powder was placed in a quartz boat located in a tube furnace and heated at 700 oC for 4 h with a heating rate of 5 oC min−1 under Ar to carbonize the organic component in Bi-MOF, the UCF@CNs@BiNs composites were obtained eventually. 2.2 Material characterizations The microstructure of UCF@CNs@BiNs was studied by Field Emission Scanning Electron Microscope (FESEM, Hitachi S-4800). And the detailed structural information of UCF@CNs@BiNs was characterized by Transmission Electron Microscope (TEM, Titan F20). Powder XRD data was provided by RIGAKU RINT-2000 (Cu Kα). Operando XRD results were acquired through Bruker AXS D8 Advance, with the investigated sample as the working electrode, K foil as the counter electrode, and 3 M KFSI in DME as the electrolyte. X-ray Photoelectron Spectroscopy (XPS) analysis was carried out on ESCALAB 250Xi for obtaining the chemical composition. The electrodes are weighted by the scale (CPA225D Sartorius) which the precision is 0.01 mg. 2.3 Electrochemical measurements The anode was prepared by mixing active materials with conductive carbon black, and sodium carboxymethyycellulose (CMC) at a weight ratio of 8:1:1 in a solution of H2O and C2H5OH. Then the slurry was pasted onto a copper foil with a mass loading of active material around1.10 (±0.20) mg cm−2, which was dried at 80 oC for over 12 h. The organic electrolyte of 3M potassium bis(fluoro-slufonyl)imide (KFSI) in dimethyl ether (DME) was prepared as previous literatures.48 Half cells of the PIBs were assembled with 2032 coin cells in glovebox under Ar with UCF@CNs@BiNs as the anode, potassium foil as the counter electrode, Whatman glass fibers as the separator, and 3M KFSI in DME as the electrolyte. Galvanostatic charge/discharge and rate performance of the battery were evaluated by Neware BTS-5. CV 3 ACS Paragon Plus Environment


curves were performed through an electrochemical workstation. 3. RESULTS AND DISCUSSIONS Figure 1 shows the schematic composition of the MOF derived UCF@CNs@BiNs, which were prepared by directly annealing the Bi-MOF formed by solvothermal reaction process. When the Bi-MOF precursor was annealed at 700 oC under Ar atmosphere, the Bi atoms agglomerated to generate Bi nanoparticles, which were uniformly dispersed in the carbon nanorods. Simultaneously, an ultra-thin carbon film was produced, wrapping the carbon nanorods matrix, which can limit most SEI films formation on the outer surface of the anode, and then avoiding the fracture of the framework during continuous charge and discharge processes, expressing an outstanding electrochemical performance.

Figure 1. Schematic illustration of the proposed mechanisms for the design of UCF@CNs@BiNs composites derived from directly annealing the Bi-MOF at 700 oC under Ar. The morphology and structure of Bi-MOF were investigated using scanning electron microscopy (SEM) (as shown in Figure 2a), which presents a rod structure. After four hours of annealing at 700 oC under Ar, the MOF-derived UCF@CNs@BiNs composites were formed, maintaining the rod framework without obvious morphology change. The SEM images showed the unique structure with an outermost layer of ultra-thin carbon film. Furthermore the as-prepared UCF@CNs@BiNs were homogeneous and fully wrapped by an ultra-thin carbon film, and numerous Bi nanoparticles were uniformly distributed in the carbon rods network. All these advantages of UCF@CNs@BiNs materials promise a superb electrochemical performance (as shown in Figure 2b–2d).

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Figure 2. a) SEM image of the Bi-MOF composites; b-d) SEM images of the UCF@CNs@BiNs composites; e-f) Low magnification TEM images of UCF@CNs@BiNs composites; g-h) High magnification TEM images of UCF@CNs@BiNs composites; i-l) Element mapping of the UCF@CNs@BiNs anode in the pristine state. Transmission electron microscopy (TEM) was applied to further explore the detailed morphology and structure. The images of the UCF@CNs@BiNs composites displayed a rod structure, with Bi nanoparticles embedded in the carbon nanorods network, and carbon rods network wre fully wrapped by an ultra-thin carbon film (as shown in Figure 2e–2f ). It is highly in accordance with the morphology characterization of SEM. High-resolution transmission electron microscopy (HRTEM) was adopted to further explore the rod structure of UCF@CNs@BiNs. As shown in Figure 2g, the thickness of carbon film was approximately ~6 nm, which was rather thin. Such a special structure of UCF@CNs@BiNs was able to increase the interface contact between Bi nanoparticles and the carbon layer. Benefiting from outermost carbon film, the SEI films formation in the outer surface, rather than on the surface of individual Bi nanoparticles, in order to reduced the repeated SEI films formation, and prevent fracture of the matrix. The particularity and stability of the structure of the UCF@CNs@BiNs materials benefit the electrochemical properties. The HRTEM image of UCF@CNs@BiNs was shown in Figure 2h, which clearly showed that the Bi nanoparticles, which were wrapped by the carbon film, were distributed in the carbon 5 ACS Paragon Plus Environment


nanorods. The obtained sample showed the characteristic spacing of 0.328 nm, corresponding to the (012) lattice planes of metallic Bi. Owing to the unique structure of UCF@CNs@BiNs, the ultra-thin carbon film covered carbon rod anode materials exhibited the ultra-long lifetime and superpower K-ion storage capabilities. The elemental mapping results (Figure 2i–2l) further confirmed the presence of Bi, C and O elements, and verified that Bi nanoparticles were uniformly distributed in the carbon rod structure. Such a unique structure with Bi nanoparticles confined by UCF@CNs framework is expected to suppress the volume change of Bi nanoparticles during repeated charge/discharge processes and improve the electrical conductivity of electrode, yielding a high performance anode. This result is consistent with the XPS results.

Figure 3. a) Power XRD patterns of UCF@CNs@BiNs composites; b-d) XPS spectra of UCF@CNs@BiNs composites; b) The high resolution Bi 4f XPS; c) The high resolution O1S XPS; d) The high resolution C1S XPS. The phase purity and crystallinity of the synthesized UCF@CNs@BiNs materials were determined by the X-ray powder diffraction (XRD) patterns, as shown in Figure 3a. The major diffraction peaks observed at 2θ = 27.16°, 37.95°, 39.61°, 48.69°, 56.02°,62.12° ,64.51° , and 70.79° belonged to the (012), (104), (110), (202), (024), (116), (122), and (214) planes of BiNs, respectively. There were no other diffraction peaks detected, suggesting the high purity of the as-synthesized UCF@CNs@BiNs crystallites. Meanwhile, the XRD patterns of 6 ACS Paragon Plus Environment

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UCF@CNs@BiNs were highly matches the standard card of Bi (PDF#85-1329). And the XRD patterns of Bi-MOF was shown in Figure S1. The surface element composition and valence state of UCF@CNs@BiNs were deciphered by X-ray photoelectron spectroscopy (XPS). The XPS verified the coexistence of Bi, O and C elements, and the additional signal of O may be derived from the adsorbed O2 on the surface of sample. Peaks of Bi 4f were presented in Figure 3b, and the binding energies (BE) located at 159.3 ± 0.1 eV and 164.6 ± 0.1 eV correspond to Bi 4f5/2 and Bi 4f7/2, respectively, indicating that the oxidation state of Bi was +3. The energy gap between Bi 4f5/2 and Bi 4f7/2 of Bi3+ was around 5.3 eV, which was in line with reported values.49 The high resolution XPS revealed that the peak intensity of O1s (Figure 3c). The peak located at 530.1 eV was indexed to Bi-O bond, and the rest two were related to the lattice oxygen and C-O bond in UCF@CNs@BiNs. The C1S (Figure 3d) XPS spectra exhibited four peaks, further validating the existence of the C-C, C-O, C=O and O-C=O bonds. The XPS results were consistent with the proposed structure and the formation mechanism of UCF@CNs@BiNs.



Figure 4. a) CV curves of the UCF@CNs@BiNs electrode from 3.0 to 0.01 V at a scan rate of 0.1 mV s−1 ; b) Charge and discharge profiles of the UCF@CNs@BiNs electrode at a current rate of 100 mA g−1 between 3.0 to 0.01 V; c) Cycling performance of UCF@CNs@BiNs electrode at a current rate of 100 mA g−1; d) Rate capacities of 8 ACS Paragon Plus Environment

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UCF@CNs@BiNs electrode at various current rates from 100 to 1000 mA g−1; e) Cycling performance of UCF@CNs@BiNs electrode at a current rate of 1000 mA g−1 after 5 cycles activated by 100 mA g−1; f) Comparison of specific capacity, cycling performance, and capacity decay of per cycle between the UCF@CNs@BiNs anode and other anodes for PIBs. To study the K-ion storage capabilities of the UCF@CNs@BiNs anodes, a series of electrochemical measurements were executed. The electrochemical performances of UCF@CNs@BiNs composites as an anode of KIB were investigated with a voltage window of 0.01–3.0 V (vs. K+/K). Figure 4a showed the initial three CV curves of the UCF@CNs@BiNs electrode at a scan rate of 0.1 mv s−1. The initial cathodic process showed a peak at 0.55 V and a sloop curve from 0.30-0.02 V during the initial cathodic scan while weakened or faded during the following cycles, which could be attributed to the potassiation of Bi and the formation of SEI layer on the surface of the electrode by electrolyte decomposition.22 The anodic process has three peaks located at 0.64, 0.71 and 1.19 V, respectively and the corresponding discharging plateaus which located at 0.75, 0.30 and 0.20 V, which could be assigned to the depotassiastion and potassiastion processes (K3Bi⇋K3Bi2⇋KBi2⇋Bi).43 The cathodic and anodic current peaks kept their shapes and intensities almost unchanged during the subsequent scans, implying the stable and reversible redox reactions. This result is highly in accordance with the operando XRD tests of UCF@CNs@BiNs (as shown in Figure S2). These results demonstrated that phase change from the pristine Bi to KBi2, K3Bi2, and K3Bi was highly reversible, and the reaction mechanism is the same as before reported.13,

43

Benefitting from the unique stucture of

UCF@CNs@BiNs anode, the carbon nanorods provides a high-speed channel for ion transport, and the SEI films can be formed stably on the surface of the carbon film after many cycles. All of these advantages further elucidating the stability of UCF@CNs@BiNs anode for potassium ion batteries. Figure 4b depicts the galvanostatic charge and discharge profiles of UCF@CNs@BiNs electrode at a current density of 100 mA g−1 with a voltage window of 0.01 –3.0 V. These profiles almost overlapped with a small polarization. In the first cycle, the UCF@CNs@BiNs electrode delivers a discharge capacity of 665.0 mA g−1. After 50 cycles, a discharge capacity of ~425.0 mA g−1 was maintained, indicating an outstanding cycle stability originating from the formation of a stable SEI films. The charge and discharge processes were referred to the potassiation and depotassiation reactions of Bi. There are three plateaus in both the charging and discharging processes, displaying three two-phase reactions (Bi ⇋ KBi2 ⇋ K3Bi2 ⇋ K3Bi), which are highly consistent with the CV curves of Figure 4a. Through the above 9 ACS Paragon Plus Environment


analysis, a high electrochemical stability of the UCF@CNs@BiNs electrode is realized. Figure 4c presents the cycling performance of UCF@CNs@BiNs over 600 cycles at a current density of 100 mA g−1. UCF@CNs@BiNs delivered a discharge capacity of ~327.0 mA g−1 after 600 cycles with an admirable average Coulombic efficiency (CE) around 98 % except initial cycles, corresponding to a capacity decay of 0.038 % per cycle. Even at a high current density of 1000 mA g−1 (Figure 4e), the UCF@CNs@BiNs anode achieved a discharge capacity of 121.0 mAh g−1 in initial cycles and presented a stable capacity, equivalent to a capacity decay of 0.036 % per cycle over 700 cycles. The remarkable electrochemical properties of UCF@CNs@BiNs illustrates a superior reversibility during charge and discharge process. The highly stable cycle performance of UCF@CNs@BiNs is might be due to the unique structure, Bi nanoparticles uniformly distributed in carbon nanorods, which can buffer the extreme volume change and is able to endure the large mechanical strains during the potassiation and depotassiation processes. In addition, the UCF@CNs matrix can allow major SEI films formation happens on the surface of carbon film, rather than on the surface of individual Bi nanoparticles, thus alleviating the pulverization of the electrode which could result in the stable properties. Furthermore, the cycle stability tests of UCF@CNs@BiNs anode at a higher current density of 200 mA g−1 and 500 mA g−1 were performed, which were illustrated in Figure S3 and Figure S5, and the corresponding charge and discharge profiles were presented in Figure S4 and Figure S6, these all expressing the considerable stable performance. Besides the excellent cycling stability, the UCF@CNs@BiNs electrode exhibited an outstanding rate performance (Figure 4d), where the current density increased step-wise from 100 to 1000 mA g−1 and then returned to 100 mA g−1. The average specific capacities for UCF@CNs@BiNs anode were 430, 360, 320, 260, and 140 mAh g−1 at the current density of 100, 300, 500, 800, 1000 mA g−1, respectively. Remarkably, when the current density returned to 100 mAh g−1, the reversible capacity can be almost fully recovered, indicating the stability of the electrode under a wide range of current densities. This superior rate performance was apparently ascribed to the architectural design of the UCF@CNs@BiNs composites as well as the high-speed channel for ion transport provided by the conductive carbon nanorods. Compared with other anodes reported for PIBs, such as hard carbon, graphite, reduced graphene oxide, and other Bi-based anodes, the UCF@CNs@BiNs exhibited one of the best electrochemical performance for PIBs, as shown in Figure 4f and Table S1.44-45, 50-52 From the above results can be drawn that the UCF@CNs@BiNs electrode is a great progress of the anode for PIBs. 10 ACS Paragon Plus Environment

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Figure 5. a) Schematic illustration of traditional Bi and the UCF@CNs@BiNs composites electrode during potassiation and depotassiation processes; b) Low magnification TEM image of UCF@CNs@BiNs composites after many cycles; c-d) High magnification TEM image of UCF@CNs@BiNs composites after many cycles. The potassiation and depotassiation processes of traditional Bi nanoparticals electrode and novel UCF@CNs@BiNs electrode were illustrated in Figure 5a. The traditional structure of Bi nanoparticles electrode was destroyed easily after many cycles. This phenomenon would cause many troubles, such as the repeated SEI films formation, rapid capacity decay, and a low coulombic efficiency. However, the rod structure of UCF@CNs@BiNs electrode can be maintained after many cycles. Such a stable structure is mainly attributed to the UCF@CNs matrix, which limits the most SEI films formation to the outer surface, rather than on the surface of individual Bi nanoparticles, thereby preventing the fracture of the matrix. And Bi nanoparticles were uniformly distributed in the carbon nanorods network, which could buffer the huge volume change of Bi nanoparticles during continuous potassiation/depotassiation processes, also can provide a high-speed channel for ion transport, all this merits guaranteed the unexceptionable electrochemical performance of the unique anode. To prove the above hypothesis, we disassembled the 2032 type coin cell with UCF@CNs@BiNs electrode after many cycles in the glove box. As displayed in the TEM 11 ACS Paragon Plus Environment


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image (Figure 5b), the morphology and structure of the UCF@CNs@BiNs present no significant change, manifesting the highly stable rod structure, which leads to the unexceptionable electrochemical properties. Also, the stable SEI film on UCF@CNs@BiNs could be observed effortlessly in the high resolution TEM image (Figure 5c and Figure S7), which is typically ~5.5 nm thick. The HRTEM image (Figure 5d) was presented to investigate the final state of the electrode. The characteristic spacing of 0.328 nm and 0.236 nm are spotted, which are indexed to the (012) and (104) crystal planes of metallic Bi, revealing the high reversibility of UCF@CNs@BiNs electrode. The morphology and the final state of UCF@CNs@BiNs electrode at discharged and charged states were also investigated by TEM and HRTEM. TEM images display that the structure of UCF@CNs@BiNs changed little no matter at discharged or charged states (Figure S8a and S9a), implying the highly stable characteristics of the electrode structure. Because of its stable structure that its excellent cycling performance is guaranteed. Meanwhile, HRTEM images prove the final state of the UCF@CNs@BiNs anode (Figure S8b and S9b). The obtained sample at the discharged state shows the characteristic spacing of 0.310 nm, corresponding to the (110) crystal plane of K3Bi (Figure S6b), implying that the Bi nanoparticals have been fully alloyed during the potassiation process. At the charged state, K3Bi has been completely dealloyed, as shown in the Figure S9b. The characteristic spacing of 0.328 nm is spotted, corresponding to the (012) crystal planes of metallic Bi. These results demonstrate the high reversibility of UCF@CNs@BiNs composites and its superior structure stability. 4. CONCLUSION In summary, UCF@CNs@BiNs, an PIBs anode reported for the first time, can deliver a high capacity of ~425 mAh g−1 and a high capacity retention of 77 % at 100 mA g−1, with an average Coulombic efficiency of 98 %, and the capacity decay of 0.038 % per cycle over 600 cycles. At a higher current density of 1000 mA g−1, a capacity decay of 0.036 % per cycle was achieved after 700 cycles. The remarkable electrochemical performance of the UCF@CNs@BiNs is ascribed to the reversible, stepwise Bi ⇋ KBi2 ⇋ K3Bi2 ⇋ K3Bi phase transitions and the stability of the structure, together with the enhancement of conductivity and volume expansion buffering offered by UCF@CNs@BiNs matrix. The unique structure is the

key

for

achieving

a

stable

electrode

architecture

for

enduring

many

potassiation/depotassiation cycles. We believe that this work could promote the study on Bi-based anode materials and potentially pave the path for the development of high performance PIBs. ACKNOWLEDGMENT 12 ACS Paragon Plus Environment

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This work was financially supported by National Natural Science Foundation of China (51672078 and 21473052) and Hunan University State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body Independent Research Project (71675004), Hunan Youth Talents (2016RS3025), and Foundation of State Key Laboratory of Coal Conversion (Grant J17-18-903). AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected].

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Control of SEI Formation for Stable Potassium-Ion Battery Anode by Bi

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