Article pubs.acs.org/JPCC
Reaction and Capacity-Fading Mechanisms of Tin Nanoparticles in Potassium-Ion Batteries Qiannan Wang,†,‡ Xinxin Zhao,§,‡ Chaolun Ni,†,‡ He Tian,† Jixue Li,† Ze Zhang,† Scott X. Mao,† Jiangwei Wang,*,† and Yunhua Xu*,§,∥ †
Center of Electron Microscopy and State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China § School of Materials Science and Engineering and Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin 300072, China ∥ Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China S Supporting Information *
ABSTRACT: In this article, we report a study of the electrochemical performance and degradation mechanism of tin (Sn) nanoparticle anodes in potassium-ion batteries (KIBs). A high capacity of 197 mAh/g was found for the Sn nanoparticles in KIBs. In situ transmission electron microscopy characterization revealed a two-step potassiation mechanism: formation of a KSn phase after full potassiation and reversible nanopore formation during the cycling of Sn nanoparticles. However, significant capacity fading occurred after a few cycles, which was caused by the severe pulverization of the Sn nanoparticles. This work offers a fundamental understanding of the reaction and degradation mechanisms of alloying-type anodes for KIBs, shedding light on the development of high-performance KIBs.
1. INTRODUCTION Increasing demands for power sources for portable devices, electric vehicles, and large-scale energy-storage applications require the development of high-performance and low-cost battery systems. Currently, the market is mainly dominated by lithium-ion batteries (LIBs); however, the sustainable application of LIBs causes serious concerns for the near future, considering the shortage and rising costs of lithium resources.1−3 This has driven a search for alternative systems as substitutes for LIBs. As a result, batteries based on earthabundant elements have attracted much attention in the past few years, including Na-ion batteries (NIBs),4,5 Mg-ion batteries,6,7 and Al-ion batteries.8 As another abundant and low-cost element,1,9,10 potassium also shows promising properties in the field of battery applications, such as a high working voltage3,11,12 and good rate capability; and some efforts have been made to develop potassium-ion batteries (KIBs). Recent studies of KIBs have mainly focused on the development of carbon-based anodes, including graphite,1,3,9,12,13 carbon nanofibers,14 hard carbon,2 and graphene.15,16 However, the large radius of potassium ions (1.38 Å) causes difficulty for insertion of potassium ions into the intercalation-type electrodes, thereby restricting the applications of these anodes in KIBs.3,12 Guided by knowledge about LIBs, the alloying mechanism might be a potential approach to overcome this problem. Nonetheless, alloying-type anodes for KIBs have been less explored so far. As a widely studied anode with excellent performance in LIBs and NIBs,17 Sn can form © XXXX American Chemical Society
several different alloys with K in the binary phase diagram, suggesting the potential application of Sn and Sn-based materials in KIBs. Recently, Sultana et al. reported a capacity of 150 mAh/g for Sn-based nanocomposite anodes in KIBs18 but with rapid capacity decay. In addition, a clear understanding of the reaction and degradation mechanisms has not been reported for Sn anodes in KIBs. Such an understanding is important for the development of Sn and other alloying-type anodes for high-performance KIBs. In this article, we report a study of the electrochemical performance and reaction and degradation mechanisms of Sn nanoparticles in KIBs using in situ transmission electron microscopy (TEM) electrochemical testing, which provides a direct way to study the fundamental science in alloying-type anodes for KIBs. Sn nanoparticles deliver a high capacity of 197 mAh/g for KIBs but decay rapidly. In situ TEM electrochemical testing reveals that the electrochemical potassiation of Sn nanoparticles proceeds in the manner of a two-step mechanism, with KSn alloy forming after full reaction whereas nanostructured pores form during depotassiation that are fully recoverable during cycling. However, significant pulverization occurs in the anodes after a few cycles, causing poor cycle stability. Received: April 24, 2017 Revised: May 16, 2017 Published: May 17, 2017 A
DOI: 10.1021/acs.jpcc.7b03837 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
2. METHODS 2.1. Material Characterization. X-ray diffraction (XRD) patterns of different specimens were recorded on a Bruker D8 Advance X-ray powder diffractometer (Bruker AXS Inc., Madison, WI) using Cu Kα radiation. Scanning electron microscopy (SEM) images were recorded on a Hitachi TDCLS-4800 scanning electron microscope (Tokyo, Japan). Both ex situ and in situ characterizations of Sn nanoparticles were conducted using an FEI Tecnai F20 field-emission-gun transmission electron microscope (TEM). The cycled electrode samples were prepared by charging/discharging the batteries to the desired voltages and holding for 5 h. After the batteries had been opened, the electrodes were washed with diethyl carbonate (DEC) and dried and sealed with parafilm inside a glovebox. 2.2. Electrochemical Measurements. Tin nanoparticles were purchased from Skyspring Nanomaterials Inc. and used as received. Tin nanoparticle electrodes were prepared by mixing the nanoparticles with carbon black and sodium alginate binder to form a slurry at a weight ratio of 60:30:10; the slurry was cast onto Cu foil using a doctor blade and dried in a vacuum oven at 100 °C overnight. Coin-type batteries were assembled with K metal as the counter electrodes, 0.8 M KPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 by volume) as the electrolyte, and Celgard 2400 membrane (Celgard LLC, Charlotte, NC) as the separator. Electrochemical performance was tested using a Land battery test station (CT2001A, Land Electronics Co., Ltd., Wuhan, China). Cyclic voltammetry (CV) curves were collected on a CHI600 electrochemical test station (CH Instruments, Inc., Austin, TX) at a scan rate of 0.015 mV/s between 0 and 2 V. 2.3. In Situ TEM Electrochemical Testing. In situ electrochemical testing of Sn nanoparticles was conducted in an FEI Tecnai F20 transmission electron microscope equipped with a Nanofactory Instruments single-tilt TEM/scanning tunneling microscopy (STM) platform. Figure S1 shows a schematic of the K−Sn nanobattery used for the in situ testing. In the nanobattery, Sn nanoparticles on a platinum (Pt) rod served as the working electrode, and bulk K metal on a tungsten (W) rod served as the counter electrode. When the nanobattery was transferred to the transmission electron microscope, some potassium oxide and potassium hydroxide (K2O + KOH) formed on the surface of K metal because of the exposure to air, acting as a solid electrolyte for K-ion transport during the in situ testing.14 After contact between the (K2O + KOH) electrolyte and Sn nanoparticles had been established, potassiation (K+ insertion) was initiated by applying a negative potential of −2 V on the Sn nanoparticles against the K metal electrode. To avoid electron-beam effects on the reaction, in situ testing was also conducted under beam-blank conditions (i.e., the electron beam was closed except for imaging during the cycles). To study the volume-change behavior after full potassiation, 10 Sn nanoparticles with different diameters were measured to minimize the measurement error. In addition, the Sn nanoparticles showed isotropic expansion upon alloying,19 further reducing the measurement error.
Figure 1. Structure of Sn nanoparticles and their electrochemical performance in KIBs. (a) Pristine Sn nanoparticles and (b) their electron diffraction pattern. (c) Cyclic voltammetry curves scanned at a rate of 0.015 mV/s. (d) Galvanostatic discharge/charge profiles tested at a current density of 20 mA/g for a Sn@C electrode in the voltage range of 0.01−2 V vs K+/K.
350 nm (Figure 1a). The electron diffraction pattern (EDP) in Figure 1b indicates that the pristine Sn nanoparticles were single crystals with a tetragonal lattice structure (β-Sn). The electrochemical performance of the Sn nanoparticles was investigated in a coin-style cell using K metal as the counter electrodes. Figure 1c shows the cyclic voltammograms of the Sn nanoparticle anodes in the first two cycles at a scan rate of 0.015 mV/s and in the voltage window of 0−2 V. A strong cathodic peak at ∼0.1 V was observed, suggesting the occurrence of an alloying reaction between Sn and K. Two cathodic waves at 0.5 and 0.75 V in the first cycle disappeared in the subsequent cycles, which is ascribed to the decomposition of the electrolyte to form solid−electrolyte interphase (SEI) films and, thereby, cause irreversible capacity loss. From the second cycle, the intensity of the main peak at 0.1 V was markedly reduced in contrast to that in the first cycle in which the SEI films formed and a small peak at 0.6 V was displayed, suggesting a two-step reaction during the alloying process. This is more clearly indicated by the two well-defined anodic peaks at 0.75 and 1.1 V during the extraction of K ions. The reversible redox behavior demonstrates good reversibility of alloying/ dealloying between Sn and K. The voltage profiles of the Sn nanoparticle anodes in the first three cycles were obtained by charging/discharging the batteries at a current density of 20 mA/g, as shown in Figure 1d. A long potassiation voltage plateau was displayed at 0.2 V, corresponding to the alloying of K with Sn. In contrast, two depotassiation plateaus at 0.75 and 1.1 V were observed during dealloying, indicating two different phase transformations during depotassiation. The potassiation and depotassiation capacities of the Sn nanoparticle anodes in the first cycle are 590 and 197 mAh/g, respectively, corresponding to a Coulombic efficiency of 33%. The irreversible capacity loss is assigned to the side reaction of SEI film formation due to the decomposition of the electrolyte. We note that the depotassiation capacity in the first cycle was close to the theoretical capacity of 226 mAh/g with the formation of KSn phase,
3. RESULTS AND DISCUSSION 3.1. Electrochemical Performance of Sn Nanoparticles. Figure 1 shows the structure of the Sn nanoparticles and their electrochemical performance as anodes in KIBs. The diameters of the pristine Sn nanoparticles ranged from 70 to B
DOI: 10.1021/acs.jpcc.7b03837 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The enlarged image in Figure 2 shows that broadened diffraction peaks arose at the position of ∼30°, which was identified to be KSn phase, revealing a phase transition from crystal Sn nanoparticles to amorphous potassiated products of KSn alloy. Upon the extraction of K ions, the crystalline structure of the β-Sn phase was fully recovered, indicating a highly reversible alloying/dealloying reaction of Sn with K, as confirmed by in situ TEM observations (shown below). Unfortunately, the Sn nanoparticle anodes underwent rapid capacity decay during cycling and failed to deliver reversible capacity after 10 cycles (Figures 1d and S2). Such capacity fading can be associated with the large volume changes occurring during the alloying/dealloying reactions, which caused severe structural pulverization of the Sn nanoparticles, as evidenced by the morphology changes of the Sn nanoparticle anodes before and after cycling (Figure S3a,b). In contrast to the large spherical Sn nanoparticles before cycling, no large nanoparticles existed in the cycled electrode, suggesting the pulverization and destruction of the nanoparticle anodes. However, the existence of Sn was revealed by the XRD data of the electrodes after cycling, as shown in Figure S3c. In situ and ex situ TEM characterizations were further conducted to clarify the degradation mechanism. 3.2. Reaction and Capacity-Fading Mechanisms of Sn Nanoparticles. Figures 3 and S4 show the reaction process of Sn nanoparticles during the first potassiation/depotassiation cycle. Like the sodiation of Sn anodes,20 potassiation of the Sn nanoparticles was dominated by a two-step mechanism, consistent with the CV measurements. That is, the crystalline Sn nanoparticles were initially potassiated to a K-poor phase with a sharp phase boundary (see Figure 3b,c, where the yellow arrows indicate the propagation direction of the phase boundary). Then, the K-poor phase was further potassiated without a phase boundary, indicating the operation of a singlephase reaction (Figure 3d−f). After the first-step potassiation, a
suggesting that one Sn atom can accept one K ion in the electrochemical reaction. XRD patterns of pristine electrodes, fully potassiated (discharged to 0 V) and fully depotassiated (charged to 2 V) electrodes from the first cycle are shown in Figure 2, as well as
Figure 2. XRD patterns of electrodes before and after cycling: (a) Sn nanoparticle powder, (b) as-prepared electrode showing the crystal structure of β-Sn phase, (c) amorphous electrode in the discharged state (0 V), and (d) recovery of the crystal structure upon charging to 2 V. (Panels c and d both correspond to the the first cycle.)
that of pure Sn nanoparticles for comparison. In Figure 2, the strong peaks of Cu are assigned to the Cu current collectors. Crystalline patterns of Sn nanoparticles were clearly observed for the pristine electrodes but disappeared upon potassiation.
Figure 3. Reaction process of Sn nanoparticles during the first potassiation/depotassiation cycle. (a) Morphology of pristine Sn nanoparticles. (b,c) First-step potassiation of Sn nanoparticles. The yellow arrows indicate the propagation direction of the phase boundary. (d−f) Second-step potassiation of Sn nanoparticles, without a phase boundary. (g−i) Depotassiation-induced nanopores in the nanoparticles. (j) Thick layer of KOH (∼10 nm) observed on the surface of potassiated nanoparticles. (k) Electron diffraction pattern of the fully potassiated nanoparticles, which can be indexed as KSn and KOH phases. (l) Electron diffraction pattern of the fully depotassiated nanoparticles, which can be indexed as the β-Sn phase. C
DOI: 10.1021/acs.jpcc.7b03837 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 4. Structural evolution of Sn nanoparticles during potassiation/depotassiation cycles. (a−h) Reversible nanopore formation during the first three cycles of depotassiation of the Sn nanoparticles. (i−k) Healing process of the nanopores during the subsequent potassiation.
volume expansion of ∼113% was obtained for the partially potassiated nanoparticles (Figure 3d), whereas the volume expansion reached ∼197% after the full potassiation. The EDP in Figure 3k shows that the potassiated nanoparticles were partially crystallized into the KSn phase with a tetragonal structure after full potassiation, in good agreement with the estimation based on the capacity (Figure 1d) and volume expansion (Table S1). We also note that a thick KOH layer (∼10 nm) grew on the surface of the potassiated nanoparticles (Figures 3j and S4), as confirmed by the wide diffraction halo from KOH in Figure 3k. Such a thick KOH layer consumes the K ions and acts as the SEI layer, which could result in a low Columbic efficiency, high impedance, and thus capacity fading. Similar behavior is expected in K−Sn batteries made with liquid electrolytes, in which SEI films can continually grow on the newly generated surface of pulverized electrodes upon potassiation/depotassiation. Because of the amorphous nature of the intermediate reaction products, it is difficult to identify the phase transformation process during the potassiation of Sn nanoparticles. However, different K−Sn alloys exhibit characteristic volume expansions with respect to pristine Sn nanoparticles, providing a possible method for estimating the K contents in Sn anodes at different stages of the reaction.19,20 Table S2 lists the theoretical volumetric expansions for several K−Sn alloys with respect to pristine Sn. The volume expansions of Sn nanoparticles with different diameters were measured by in situ TEM and are summarized in Table S1. It can be seen that the volume expansion of the potassiated nanoparticles was about 110% after the phase boundary swept the whole nanoparticle (i.e., the first-step potassiation), close to the theoretical value for the K4Sn9 phase, whereas it was between 167% and 197% after the full potassiation, indicating the formation of the KSn phase. Some intermediate KxSn phases might have formed during the second-step potassiation. However, they would be difficult to identify because of their amorphous nature and the lack of available diffraction data.
Figure 3g−l shows the dealloying of the potassiated nanoparticles. The depotassiation was conducted under beamblank conditions to avoid beam irradiation of the K+ extraction, and a positive potential of +1 V was applied to the KSn nanoparticles with respect to the K metal side. Upon depotassiation, nanoscale pores formed inside the KSn nanoparticles, nucleating from the contact region between the nanoparticles and K metal and then gradually propagating toward the Pt side (Figure 3g,h). With the extraction of the K ions, more nanopores nucleated, with some of them growing larger and larger (Figure 3i), and the KSn phase was gradually transformed back to the crystal β-Sn phase, as indicated by the EDP in Figure 3l and the XRD pattern in Figure 2. After full depotassiation, limited volume shrinkage of the Sn nanoparticles occurred because of the formation of numerous nanopores (Figure 3i). Similar nanoporous structures have also been observed in other battery systems, such as Li−Ge,21 Li− Sn,19 and Li−Ga.22 The formation of the porous structure during depotassiation can be attributed to the high transport rate of K+, as well as the fast local aggregation of vacancies produced by the rapid extraction of K+. After full depotassiation, the diameter of the nanoparticles did not change much because of the existence of numerous nanopores. Figure 4 shows the dynamic electrochemical behavior of Sn nanoparticles during the potassiation/depotassiation cycles. Repeated insertion and extraction of K ions caused reversible volume changes of the Sn nanoparticles, along with recoverable nanopores that were generated during depotassiation and then completely healed during the subsequent potassiation (Figure 4a−h). As shown in Figure 4i−k, K+ insertion caused the volume increase of the Sn ligaments between two nanopores, which squeezed into the open space of the nanopores. As a result, the nanopores were gradually distorted, accompanied with the size shrank (Figure 4i−k). After full potassiation, the nanopores were healed by the newly formed K−Sn alloys (Figure 4l), suggesting a partial self-healing ability for Sn anodes in KIBs. Because the nanopores formed during D
DOI: 10.1021/acs.jpcc.7b03837 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 5. Pulverization of Sn nanoparticles after cycling. (a,b) Pulverization of Sn nanoparticles after beam-blank depotassiation. (c−f) Ex situ characterization of the anodes after 34 cycles in KIBs.
nanoparticles were found to deliver a capacity of 197 mAh/g, but this capacity decayed rapidly. In situ TEM studies revealed the reaction mechanism of Sn nanoparticle anodes during potassiation/depotassiation cycles, involving two-step potassiation of Sn nanoparticles, the formation of the KSn phase after full reaction, the reversible formation of nanopores, and pulverization. Significant capacity fading of the Sn nanoparticle anodes was found to occur after few cycles, which was caused by the severe pulverization of the Sn nanoparticles. The full reaction map of Sn nanoparticles in KIBs can be summarized as follows
depotassiation can provide some space to accommodate further volume expansion, the volume increase of the depotassiated nanoparticles can be alleviated in subsequent potassiations as compared with that of the pristine Sn nanoparticles in the first potassiation (Figure 4).The unique behavior of the two-step reaction and nanopore healing in K−Sn batteries can have several consequences, including (1) alleviation of the stress/ strain concentration at the reaction front and thus of alloyinginduced crack generation;19,20 (2) accommodation of the volume expansion by the open space in the nanopores after the first cycle;23 and (3) self-healing of the nanoporous structure.21 These benefits can contribute to robust cycle stability of anodes in other battery systems, such as Li−Ge,21 Li−Sn,19 and Li− Ga.22 Even with these benefits, however, the K−Sn batteries tended to show a poor cycle performance as indicated by the electrochemical results (Figure S2). The rapid capacity decay in the coin cells was induced by the pulverization of the Sn anodes during K-ion insertion/extraction cycles, during which huge volumetric expansion and contraction destroyed the structure of the Sn nanoparticles and SEI films formed in the previous cycle, generating a fresh contact surface exposed to electrolyte. Figure 5a,b shows an example of the crack-induced pulverization of a Sn nanoparticle after depotassiation under beam-off conditions. The repetitive breaking of the structure and formation of the SEI consume electrolyte and electronically isolate the pulverized Sn particles, leading to mechanical degradation of the electrodes and a rapid decay of battery performance (Figure S1). Ex situ characterization of the Sn anodes after 10 cycles in a KIB cell further confirmed the pulverization of the Sn nanoparticles. As shown in Figure 5c,d, large holes existed in the cycled anode without any bulk-size Sn nanoparticles, whereas the high-resolution TEM image in Figure 5e reveals that the Sn anodes were pulverized into atomic clusters of less than 5 nm. The EDP in Figure 5f shows only the diffraction halos from carbon black with a disordered structure, further suggesting the severe pulverization of the Sn nanoparticles after cycling.
It should be pointed out that the partially potassiated intermediate K−Sn phase observed in the in situ experiments might be different from that formed in real batteries because the phase behavior is highly influenced by kinetic factors, such as experimental configurations.20 Nonetheless, our results offer new understanding of the fundamental science in KIBs, shedding light on the rational design of novel Sn-based anodes for high-performance KIBs.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b03837. Schematic of the experimental setup, cycling stability of the Sn nanoparticle anodes, morphologies of the Sn electrodes before and after cycling and XRD pattern of a Sn electrode after 10 cycles, beam-blank potassiation of Sn nanoparticles showing a two-step reaction process and the growth of surface KOH, volume expansions of Sn nanoparticles of different sizes, and theoretical volumetric expansions of different KxSn phases with respect to pristine Sn nanoparticles(PDF)
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4. CONCLUSIONS Through electrochemical performance testing and direct structural characterization in cycles, we revealed in this work the reaction and degradation mechanisms in K−Sn batteries. Sn
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
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DOI: 10.1021/acs.jpcc.7b03837 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Jiangwei Wang: 0000-0003-1191-0782 Author Contributions ‡
Q.W., X.Z., and C.N. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.W. and Y.X. acknowledge the support of the Chinese 1000Youth-Talent Plan. Y.X. acknowledges support from the National Natural Science Foundation of China (Grant 51672188) and Tianjin Natural Science Foundation (Grant 16JCYBJC40900). Z.Z. acknowledges support from the National Natural Science Foundation of China (Grants 11234011 and 11327901). This work was also supported by the Fundamental Research Funds for the Central Universities (Grant 2017QNA4008).
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