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Recent progress of analysis techniques for silicon-based anode of lithium-ion batteries
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Review Article Recent progress of analysis techniques for silicon-based anode of lithium-ion batteries
Yeonguk Son, Jaekyung Sung, Yoonkook Son and Jaephil Cho∗ Various cutting-edge analysis techniques have been developed to investigate experimental phenomena on silicon-based anode for lithium-ion batteries. We classified phenomena on Si-based anode as three aspects; volume expansion, SEI formation, and phase transformation, and reviewed analysis techniques and results concerning these phenomena. Address Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50, UNIST-gil, Ulsan 44919, Republic of Korea ∗
Corresponding author: Cho, Jaephil ([email protected])
Current Opinion in Electrochemistry 2017, 6:77–83 This review comes from a themed issue on Batteries and Supercapacitors Edited by Seung Mo Oh For a complete overview see the Issue and the Editorial Available online 16 October 2017 https://doi.org/10.1016/j.coelec.2017.10.005 2451-9103/© 2017 Published by Elsevier B.V.
Introduction Regardless of numerous studies proposing successful strategies on silicon (Si)-based anode for lithium-ion batteries (LIBs), usage of Si anode materials in commercial LIBs is still limited because of poor electrochemical results based on the test conditions with commercial level, such as areal capacity over 3.0 mAh/cm2 , electrode density over 1.6 g/cc and minimum amount of binder materials under 4 wt%. Although there have been some worthy works showing comparable stable cycling properties with commercial graphite anode, there has not been the work overwhelming the performance of commercial graphite anode in long-term full cell test over 1000 cycles [1–4]. Therefore, understanding the phenomena on Sibased anode during charging/discharging process, which induces a poor electrochemical performance, is required to develop advanced strategies overcoming its problems. Analyzing what happens in electrode should be tremendously challenging technique because it is very tough and complex work to transfer the sample, taken from elecwww.sciencedirect.com
trode in cycled lithium (Li)-ion cells to analysis equipment without exposure to oxygen, water or any contaminating substance [5]. Even if it was carefully transferred to the equipment, organic materials in electrode, such as binder and solid electrolyte interphase (SEI), could be damaged during analysis like electron microscopy with a high energy source [5]. Therefore, the developments of analyzing techniques have been mostly focused on in situ techniques such as in situ transmission electron microscopy (TEM) with limited use of accelerating voltage, X-ray diffraction (XRD), Raman, nuclear magnetic resonance (NMR) and so on rather than ex situ methods [6]. However, there are even some differences between in situ systems and real LIBs. For example, in in situ TEM, volatile electrolyte could not be used because of vacuum condition. Thus, ionic liquid or solid electrolyte is usually utilized [7]. Nevertheless, the conductivities of these electrolytes are lower than those of conventional carbonate electrolytes and the components of SEI would be changed. Therefore, the developing in situ measurement system toward real LIB conditions or resolving the problem of ex situ measurement system such as electrode transferring must progress constantly. Moreover, the results of the analysis should be evaluated with scientific hypothesis and logics. In this context, herein we summarized the cutting-edge of analyzing technology for Si-based anodes in LIBs during past two years. The scope of this review will cover whole phenomena on Si electrode and bring positive effect for researchers who are working on development of Si-based anodes.
Critical issues in analyzing Si-based anode The origins of fading mechanism in Si-based anodes are related to large volume change and SEI formation. Large volume change induces pulverization and cracking; newly revealed and cracked surface cause new SEI formation, and repeated cycling creates growing SEI layers thicker (Figure 1A). These phenomena produced by large volume change and SEI formation cause active material loss, which is physically or electrochemically separated from the conduction pathway; it is dead Si which could not contribute to capacities [8,9]. Another important issue in Sibased anode is a phase transformation. Depending on the atomic structures, the mechanism of phase transformation could be different and thus electrochemical performance could be varied [10,11]. Here we classified analyzing aspects of the phenomena on Si electrode into three categories; volume change, SEI formation and phase transformation (Figure 1B). Current Opinion in Electrochemistry 2017, 6:77–83
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Figure 1
(A) Schematic view of fading mechanism of Si-based anode (B) summary of recent analyzing methods and results regarding phenomena on Si-based anode.
Volume change and mechanical properties Mechanical properties with regard to volume change during charging/discharging are the most critical issues in Sibased anode because all the fading mechanisms of Sibased anode are directly or indirectly related to them. Hence, there have been various approaches to observe the mechanical properties relating to volume change of Si-based anode. The simplest method to observe volume expansion is measuring the thickness of the electrode after cycling. This process could only be conducted in a dry room or glove box for safety, and to avoid side reactions. However, it could merely inform extent of volume change of electrode rather than the effect of Si volume expansion, properties of expanded materials or damages on vicinities of active materials. Therefore, it is needed to develop the technologies of volume change measurement and characterization of active material itself. Lately, in situ TEM analysis technology has been evidently improved. Beyond the observation of enlarging particle and increasing crack formation, researchers try to reveal the comprehensive effect and properties of Si expansion. Lee et al. reported the studies on the direction of Si expansion in compressed condition and correlation Current Opinion in Electrochemistry 2017, 6:77–83
between fraction resistance and clamping [12• ]. They exploited Si nanopillar synthesized by Si wafer etching. In the ex situ scanning electron microscopy (SEM) and in situ TEM analysis, they revealed 1 1 0 directional expansion of Si nanopillar was faster than the 1 0 0 directional expansion in unclamped sample, and further 1 0 0 directional expansion was shown while the 1 1 0 directional expansion was clamped (Figure 2A). That is, the direction of Si expansion is favored to unclamped area which is empty space rather than clamped area. They also discovered that the fracture resistance is improved by mechanical clamping thus it could enable bigger critical size of Si without fracture. Wang et al. reported strength and ductility of Si and lithiated Si through nanoindentation by in situ TEM [13• ]. They used Si nanowire and thin film as active materials and provided in situ TEM results of broken Si and stretched Li silicide when partially lithiated Si nanowire is compressed (Figure 2B). They also calculated the fracture toughness and energy of lithiated Si as a function of Li concentration from nanoindentation and those calculated fracture properties are considerably increased with increase of Li content (Figure 2B). Subsequently, they demonstrated that pristine Si was brittle, whereas Li silicide revealed ductile features. www.sciencedirect.com
Recent progress of analysis techniques for silicon-based anode of lithium ion batteries Son et al.
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Figure 2
Analyzing results of volume changes and mechanical properties on Si-based anode (A) anisotropic volume expansion of Si in clamped condition, SEM images of (a) pristine Si nanopillar and (b) Si nanopillar after lithiation, (c) expansions of Si nanopillar depending on direction, (d) fracture ratios depending on positions of compressing walls, (e) population of the fracture location depending on the angle of cracks (B) increased fracture toughness in lithiated Si (a) TEM images of bent partially lithiated Si nanowires, (b) finite element simulated elastic-plastic deformation, (c) indentation load depending on Li contents in Lix Si, 1–6 point shows experimental measurement, blue line represent the upper load limit where massive cracking occurred, black line represent the lower limit below where no cracking occurred. The dashed line was interpolated from the data, (d) facture toughness and energy depending on Li contents in Lix Si. (C) microstructural changes occurring in graphite-Si electrode (a) the relative volumetric change of graphite–silicon electrode was expressed by divergence (from left to right, SOCs of electrode are 0, 17, 46, 92%. Scale bar length: 50 μm), (b) electrode expansion based on mean of cumulated divergence (red) and electrode thickness (black). (c) Volume factions in the pristine and the lithiated electrode. The black values are gained based on the chemical composition of the electrode. The blue values are gained based on a segmentation of the tomographic data in the three phases. Reprinted with permission from Ref. [12• –14•• ]. Copyright 2015 Nature Publishing Group.; Copyright 2016 Nature Publishing Group.
Another leading edge of analyzing technique for volume expansion is operando tomography. Pietsch et al. reported microstructural changes of graphite and graphite–silicon blending electrodes (weight ratio of graphite and siliwww.sciencedirect.com
con = 75 (graphite): 25 (silicon) and active material ratio in electrode is 80 wt%) [14•• ]. In this work, the blending electrode showed expansion of 35% and reduction of the porosity by 8% (Figure 3C). This technique enables Current Opinion in Electrochemistry 2017, 6:77–83
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Figure 3
Analyzing results of SEI formation on Si-based anode (A) SEI compensation occurs during lithiation process (a) AFM images of SEI layers during (a–c) the first cycle, (c–e) the second cycle, and (e–g) the third cycle. (B) SEI growth (inorganic → organic) and SEI accumulation in pores, results of 13 C, 7 Li, and 19 F ssNMR with (a) 13 C EC (blue) and (b) 13 C DMC (purple) (C) electrochemically isolated Si nanoparticle by SEI formation, STEM-EELS element mapping of Si electrodes cycled at 30% of their theoretical capacity (1200 mAh/g) (a) after the first lithiation, unlithiated part covered by LiF, (b) after the first lithiation, cristalline Si/LixSi core-shell part covered by ∼20 nm thick carbonates layers and LiF particles. (c) After 100th delithiation, delithiated Si particles trapped in LiF matrix, (d) after 100th delithiation, delithiated Si particles covered by thick (∼100 nm) carbonate layer, (e) low-loss EEL spectra on 1 and 2 sites showing low lithium content on carbonate layer after extended cycling. Reprinted with permission from Ref. [17• –19•• ]. Copyright 2016 American Chemical Society.
to quantify the microstructural changes taking place in an electrode during lithiation/delithiation and it is expected to be useful for analyzing porous Si electrode.
Therefore, analyzing SEI layer as well as volume expansion is critical to establish the new strategies for Si-based anode materials.
SEI formation
Recently, one of the excellent approaches was focused on providing direct observation of SEI degradation by using in situ AFM [17• ]. In the work, the authors revealed significant difference between center and edge parts of Si active material, and SEI formation was preferred where extensive cracking occurs; that is the edge part. The crack formed during the first lithiation was not covered at low potential. At subsequent lithiation, the SEI layers on cracked site were developed (Figure 3A). Although the LIB academic circles have debated and predicted the place and timing on SEI formation for long time, it was mostly only prediction and indirect evidences.
Anode materials which have lower working potential than the reduction potential of carbonate electrolyte are not able to avoid formation of SEI layers [15]. Commercial graphite anode retains good cycling life in carbonate electrolyte, though SEI formation slightly reduces initial coulombic efficiency [16]. In contrast, Si anode suffers gradual capacity fading by continuous SEI formation during cycling because volume expansion brings new interface between Si and electrolyte [8]. Hence, combined effect of volume expansion and SEI formation; continuous SEI thickening causes electrochemically isolated Si part. Current Opinion in Electrochemistry 2017, 6:77–83
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Recent progress of analysis techniques for silicon-based anode of lithium ion batteries Son et al.
This work proved the actual and direct proof of SEI formation with experimental observation. On the other hand, material composition and distribution are additional inquisitive point in SEI studies. Michan et al. utilized 7 Li, 19 F, and 13 C solid state NMR (ssNMR), FIB, and SEM to investigate SEI growth and accumulation of SEI in the pores of electrode [18• ]. They compared the three electrodes at 1st, 30th, and 60th cycle. From the 19 F ssNMR, LiF peak intensity at 1st cycle was nearly maintained until 60th cycle, which means LiF species primarily formed at 1st cycle. On the contrary, 13 C ssNMR presented SEI including organic species such as –OCH2 CH2 O–, CH2 O, and functional groups CH3 R, RCH2 R, and CH3 CH2 R showing materials of ROCO2 Li, oligomeric species, Li ethylene carbonate, Li butylene dicarbonate, RCO2 Li, HCO2 Li, Li2 CO3 , and Li methyl carbonate. The signal intensities of these materials were increased after 30th cycle; it means organic SEI layers are grown. They also confirmed SEI formation induced the disappearing pore as cycle goes, by SEM images of FIB cross sections. Although NMR studies provided extensive view of the SEI components, it could not afford spatial information on particular particles. Boniface et al. demonstrated electron energy-loss spectroscopy (EELS) in a scanning trans-
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mission electron microscopy (STEM) mapping SEI components with resolutions down to 5 nm [19•• ]. Figure 3C shows the color mapping of SEI components on Si electrodes cycled at 30% of state of charge (SOC). In electrodes after both 1st and 100th cycle, LiF and carbonate components were detected and the thickness of carbonate layer was increased from 15 ∼ 25 nm up to ∼100 nm. They also revealed LiF-rich SEI layers was advantageous for lithiation compared to carbonate-rich SEI layers, which implies electrochemical isolation could be caused by carbonate buildups.
Phase transformation In precedent two sections, we focused on mechanical change (volume expansion) and chemical change (SEI formation) issues of Si-based electrode. From now on, this section would regard atomic change of Si-based electrode, in other words, phase transformation which could reflect kinetics and reversibility of Si and Lix Si. Formerly, structural change of the lithiation of Si was investigated by several methods such as in situ XRD, TEM, and NMR [10,20,21•• ]. From those, singular properties about phase transformation, such as amorphization of crystalline Si during lithiation, crystalline phase of Li15 Si4 , anisotropic lithiation into crystalline Si and etc., were introduced and studied so far. However, there have been only few outstanding outcomes in terms of sophisticatedly quan-
Figure 4
Analyzing results of phase transformation on Si-based anode (A) lithiation degree and crystallinity of Si (a) STEM-EELS Lix Si alloy composition mapping of Si electrodes after 10th lithiation to 1200 mAh/g (b) STEM-EELS Si crystallinity (plasmon peak position) mapping of Si electrodes (B) remained lithium in Lix Si after delithiation, the scattering length density profiles after (a) the second lithiation and (b) the second delithiation as function of time and distance from the interface (c-Si (green), the electrolyte (yellow), surface lithiation (red/dark-red), and deep lithiation (red-yellow)). Reprinted with permission from Ref. [19•• ,22•• ]. Copyright 2016 American Chemical Society. www.sciencedirect.com
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titative and nanoscale-resolution analysis. The STEMEELS method which is introduced in previous section could be also proper technique to investigate phase transformation of Si electrode [19•• ]. With this technique, lithiation degree and crystallinity of Si can be observed; hence it could discover electrochemically isolated Si part by dense SEI layers (Figure 4A). Another technique is neutron reflectivity which enables non-destructive and quantitative analysis with nanometer-resolution scale. Seidlhofer et al. demonstrated in situ neutron reflectivity to investigate the alloying of bulk crystalline silicon [22•• ]. They utilized 1 cmsized single crystal Si and observed extent of lithiation into Si layer depending on the depth and time. The Li silicide layer was subdivided into two distinct parts; highly lithiated skin region (x ≈ ≈ 2.5 in Lix Si) and partially lithiated growth region (x ≈ ≈ 0.1 in Lix Si) (Figure 4B). After fully delithiation, the Li content in skin region remained as x 1.1 in Lix Si. The thickness of skin layer maintained, however, one of the growth regions increased as cycle goes (Figure 4B). It means there are inactivated and irreversible region in thick Si anode.
Acknowledgments This work was supported by the IT R&D program of MOTIE/KEIT (Development of Li-rich Cathode and Carbon-free Anode Materials for High Capacity/High Rate Lithium Secondary Batteries, 10046306).
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: •
1.
Jin Y, Li S, Kushima A, Zheng XQ, Sun YM, Xie J, Sun J, Xue WJ, Zhou GM, Wu J, et al.: Self-healing SEI enables full-cell cycling of a silicon-majority anode with a coulombic efficiency exceeding 99.9%.. Energy Environ Sci 2017, 10:580–592.
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Ko M, Chae S, Ma J, Kim N, Lee HW, Cui Y, Cho J: Scalable synthesis of silicon-nanolayer-embedded graphite for high-energy lithium-ion batteries. Nat. Energy 2016:1.
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Li XL, Yan PF, Xiao XC, Woo JH, Wang CM, Liua J, Zhang JG: Design of porous Si/C-graphite electrodes with long cycle stability and controlled swelling. Energy Environ Sci 2017, 10:1427–1434.
4.
Li YZ, Yan K, Lee HW, Lu ZD, Liu N, Cui Y: Growth of conformal graphene cages on micrometre-sized silicon particles as stable battery anodes. Nat Energy 2016:1.
5.
Chen D, Indris S, Schulz M, Gamer B, Monig R: In situ scanning electron microscopy on lithium-ion battery electrodes using an ionic liquid. J Power Sources 2011, 196:6382–6387.
6.
Yang J, Muhammad S, Jo MR, Kim H, Song K, Agyeman DA, Kim YI, Yoon WS, Kang YM: In situ analyses for ion storage materials. Chem Soc Rev 2016, 45:5717–5770.
7.
Liu XH, Huang JY: In situ TEM electrochemistry of anode materials in lithium ion batteries. Energy Environ Sci 2011, 4:3844–3860.
8.
Dupre N, Moreau P, De Vito E, Quazuguel L, Boniface M, Bordes A, Rudisch C, Bayle-Guillemaud P, Guyomard D: Multiprobe study of the solid electrolyte interphase on silicon-based electrodes in full-cell configuration. Chem Mater 2016, 28:2557–2572.
9.
Wu H, Cui Y: Designing nanostructured Si anodes for high energy lithium ion batteries. Nano Today 2012, 7:414–429.
Conclusion In conclusion, we reviewed the unique characterizations of Si-based anodes according to three aspects; 1) volume change extended to mechanical properties, 2) SEI formation related to surface reaction on Si electrode, and 3) phase transformation which is atomic rearrangement. For further studies, analysis of Si-based anode should be focused on evaluation of higher scales; cells and electrodes which include effects on materials, binder materials, conductive materials, current collector, and electrolyte rather than that of only materials, for instance, in situ SEM or TEM observing cross sectional electrode expansion rather than just particle expansion, or in situ chemical element analysis detecting the exhaustion of electrolyte as well as SEI formation. These fundamental understandings of phenomena on Sibased anode are wished to finally derive the solution enabling wide use of Si-based anode in commercial LIBs. In detail, intrinsic expansion of Si active material itself could not be dynamically controlled in atomic scale, whereas the expansion in electrode level would rather be considered. Hence, the bad effects from electrode expansion have been controlled by pore structure, buffer matrix or binder materials which inhibit pulverization and cracking. Although we have to admit that SEI formation is inevitable due to intrinsic reduction potentials, it is still necessary to develop effective electrolyte making robust SEI layer. In addition, atomic design of Si-based materials should provide faster kinetics and higher reversibility founded on phase transformation studies. We believe and hope this review would be a great help to researchers developing Si-based anode for LIBs. Current Opinion in Electrochemistry 2017, 6:77–83
Paper of special interest. Paper of outstanding interest.
••
10. McDowell MT, Lee SW, Harris JT, Korgel BA, Wang CM, Nix WD, Cui Y: In situ TEM of two-phase lithiation of amorphous silicon nanospheres. Nano Lett 2013, 13:758–764. 11. McDowell MT, Lee SW, Nix WD, Cui Y: 25th anniversary article: understanding the lithiation of silicon and other alloying anodes for lithium-ion batteries. Adv Mater 2013, 25:4966– 4984. 12. Lee SW, Lee HW, Ryu I, Nix WD, Gao HJ, Cui Y: Kinetics and • fracture resistance of lithiated silicon nanostructure pairs controlled by their mechanical interaction. Nat Commun 2015:6. 13. Wang XJ, Fan FF, Wang JW, Wang HR, Tao SY, Yang A, Liu Y, • Chew HB, Mao SX, Zhu T, et al.: High damage tolerance of electrochemically lithiated silicon. Nat Commun 2015:6. 14. Pietsch P, Westhoff D, Feinauer J, Eller J, Marone F, •• Stampanoni M, Schmidt V, Wood V: Quantifying microstructural dynamics and electrochemical activity of graphite and silicon-graphite lithium ion battery anodes. Nat Commun 2016:7. 15. An SJ, Li JL, Daniel C, Mohanty D, Nagpure S, Wood DL: The state of understanding of the lithium-ion-battery graphite solid electrolyte interphase (SEI) and its relationship to formation cycling. Carbon 2016, 105:52–76. 16. Xiong DJ, Burns JC, Smith AJ, Sinha N, Dahn JR: A high precision study of the effect of vinylene carbonate (VC) additive in Li/graphite cells. J Electrochem Soc 2011, 158:A1431–A1435. www.sciencedirect.com
Recent progress of analysis techniques for silicon-based anode of lithium ion batteries Son et al.
17. Kumar R, Tokranov A, Sheldon BW, Xiao XC, Huang ZQ, Li CZ, • Mueller T: In situ and operando investigations of failure mechanisms of the solid electrolyte interphase on silicon electrodes. ACS Energy Lett 2016, 1:689–697.
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20. Hatchard TD, Dahn JR: In situ XRD and electrochemical study of the reaction of lithium with amorphous silicon. J Electrochem Soc 2004, 151:A838–A842.
18. Michan AL, Divitini G, Pell AJ, Leskes M, Ducati C, Grey CP: Solid • electrolyte interphase growth and capacity loss in silicon electrodes. J Am Chem Soc 2016, 138:7918–7931.
21. Ogata K, Salager E, Kerr CJ, Fraser AE, Ducati C, Morris AJ, •• Hofmann S, Grey CP: Revealing lithium-silicide phase transformations in nano-structured silicon-based lithium ion batteries via in situ NMR spectroscopy. Nat Commun 2014:5.
19. Boniface M, Quazuguel L, Danet J, Guyomard D, Moreau P, •• Bayle-Guillemaud P: Nanoscale chemical evolution of silicon negative electrodes characterized by low-loss STEM-EELS. Nano Lett 2016, 16:7381–7388.
22. Seidlhofer BK, Jerliu B, Trapp M, Huger E, Risse S, Cubitt R, •• Schmidt H, Steitz R, Ballauff M: Lithiation of crystalline silicon as analyzed by operando neutron reflectivity. ACS Nano 2016, 10:7458–7466.
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Current Opinion in Electrochemistry 2017, 6:77–83
Review Article Recent progress of analysis techniques for silicon-based anode of lithium-ion batteries
Yeonguk Son, Jaekyung Sung, Yoonkook Son and Jaephil Cho∗ Various cutting-edge analysis techniques have been developed to investigate experimental phenomena on silicon-based anode for lithium-ion batteries. We classified phenomena on Si-based anode as three aspects; volume expansion, SEI formation, and phase transformation, and reviewed analysis techniques and results concerning these phenomena. Address Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50, UNIST-gil, Ulsan 44919, Republic of Korea ∗
Corresponding author: Cho, Jaephil ([email protected])
Current Opinion in Electrochemistry 2017, 6:77–83 This review comes from a themed issue on Batteries and Supercapacitors Edited by Seung Mo Oh For a complete overview see the Issue and the Editorial Available online 16 October 2017 https://doi.org/10.1016/j.coelec.2017.10.005 2451-9103/© 2017 Published by Elsevier B.V.
Introduction Regardless of numerous studies proposing successful strategies on silicon (Si)-based anode for lithium-ion batteries (LIBs), usage of Si anode materials in commercial LIBs is still limited because of poor electrochemical results based on the test conditions with commercial level, such as areal capacity over 3.0 mAh/cm2 , electrode density over 1.6 g/cc and minimum amount of binder materials under 4 wt%. Although there have been some worthy works showing comparable stable cycling properties with commercial graphite anode, there has not been the work overwhelming the performance of commercial graphite anode in long-term full cell test over 1000 cycles [1–4]. Therefore, understanding the phenomena on Sibased anode during charging/discharging process, which induces a poor electrochemical performance, is required to develop advanced strategies overcoming its problems. Analyzing what happens in electrode should be tremendously challenging technique because it is very tough and complex work to transfer the sample, taken from elecwww.sciencedirect.com
trode in cycled lithium (Li)-ion cells to analysis equipment without exposure to oxygen, water or any contaminating substance [5]. Even if it was carefully transferred to the equipment, organic materials in electrode, such as binder and solid electrolyte interphase (SEI), could be damaged during analysis like electron microscopy with a high energy source [5]. Therefore, the developments of analyzing techniques have been mostly focused on in situ techniques such as in situ transmission electron microscopy (TEM) with limited use of accelerating voltage, X-ray diffraction (XRD), Raman, nuclear magnetic resonance (NMR) and so on rather than ex situ methods [6]. However, there are even some differences between in situ systems and real LIBs. For example, in in situ TEM, volatile electrolyte could not be used because of vacuum condition. Thus, ionic liquid or solid electrolyte is usually utilized [7]. Nevertheless, the conductivities of these electrolytes are lower than those of conventional carbonate electrolytes and the components of SEI would be changed. Therefore, the developing in situ measurement system toward real LIB conditions or resolving the problem of ex situ measurement system such as electrode transferring must progress constantly. Moreover, the results of the analysis should be evaluated with scientific hypothesis and logics. In this context, herein we summarized the cutting-edge of analyzing technology for Si-based anodes in LIBs during past two years. The scope of this review will cover whole phenomena on Si electrode and bring positive effect for researchers who are working on development of Si-based anodes.
Critical issues in analyzing Si-based anode The origins of fading mechanism in Si-based anodes are related to large volume change and SEI formation. Large volume change induces pulverization and cracking; newly revealed and cracked surface cause new SEI formation, and repeated cycling creates growing SEI layers thicker (Figure 1A). These phenomena produced by large volume change and SEI formation cause active material loss, which is physically or electrochemically separated from the conduction pathway; it is dead Si which could not contribute to capacities [8,9]. Another important issue in Sibased anode is a phase transformation. Depending on the atomic structures, the mechanism of phase transformation could be different and thus electrochemical performance could be varied [10,11]. Here we classified analyzing aspects of the phenomena on Si electrode into three categories; volume change, SEI formation and phase transformation (Figure 1B). Current Opinion in Electrochemistry 2017, 6:77–83
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Figure 1
(A) Schematic view of fading mechanism of Si-based anode (B) summary of recent analyzing methods and results regarding phenomena on Si-based anode.
Volume change and mechanical properties Mechanical properties with regard to volume change during charging/discharging are the most critical issues in Sibased anode because all the fading mechanisms of Sibased anode are directly or indirectly related to them. Hence, there have been various approaches to observe the mechanical properties relating to volume change of Si-based anode. The simplest method to observe volume expansion is measuring the thickness of the electrode after cycling. This process could only be conducted in a dry room or glove box for safety, and to avoid side reactions. However, it could merely inform extent of volume change of electrode rather than the effect of Si volume expansion, properties of expanded materials or damages on vicinities of active materials. Therefore, it is needed to develop the technologies of volume change measurement and characterization of active material itself. Lately, in situ TEM analysis technology has been evidently improved. Beyond the observation of enlarging particle and increasing crack formation, researchers try to reveal the comprehensive effect and properties of Si expansion. Lee et al. reported the studies on the direction of Si expansion in compressed condition and correlation Current Opinion in Electrochemistry 2017, 6:77–83
between fraction resistance and clamping [12• ]. They exploited Si nanopillar synthesized by Si wafer etching. In the ex situ scanning electron microscopy (SEM) and in situ TEM analysis, they revealed 1 1 0 directional expansion of Si nanopillar was faster than the 1 0 0 directional expansion in unclamped sample, and further 1 0 0 directional expansion was shown while the 1 1 0 directional expansion was clamped (Figure 2A). That is, the direction of Si expansion is favored to unclamped area which is empty space rather than clamped area. They also discovered that the fracture resistance is improved by mechanical clamping thus it could enable bigger critical size of Si without fracture. Wang et al. reported strength and ductility of Si and lithiated Si through nanoindentation by in situ TEM [13• ]. They used Si nanowire and thin film as active materials and provided in situ TEM results of broken Si and stretched Li silicide when partially lithiated Si nanowire is compressed (Figure 2B). They also calculated the fracture toughness and energy of lithiated Si as a function of Li concentration from nanoindentation and those calculated fracture properties are considerably increased with increase of Li content (Figure 2B). Subsequently, they demonstrated that pristine Si was brittle, whereas Li silicide revealed ductile features. www.sciencedirect.com
Recent progress of analysis techniques for silicon-based anode of lithium ion batteries Son et al.
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Figure 2
Analyzing results of volume changes and mechanical properties on Si-based anode (A) anisotropic volume expansion of Si in clamped condition, SEM images of (a) pristine Si nanopillar and (b) Si nanopillar after lithiation, (c) expansions of Si nanopillar depending on direction, (d) fracture ratios depending on positions of compressing walls, (e) population of the fracture location depending on the angle of cracks (B) increased fracture toughness in lithiated Si (a) TEM images of bent partially lithiated Si nanowires, (b) finite element simulated elastic-plastic deformation, (c) indentation load depending on Li contents in Lix Si, 1–6 point shows experimental measurement, blue line represent the upper load limit where massive cracking occurred, black line represent the lower limit below where no cracking occurred. The dashed line was interpolated from the data, (d) facture toughness and energy depending on Li contents in Lix Si. (C) microstructural changes occurring in graphite-Si electrode (a) the relative volumetric change of graphite–silicon electrode was expressed by divergence (from left to right, SOCs of electrode are 0, 17, 46, 92%. Scale bar length: 50 μm), (b) electrode expansion based on mean of cumulated divergence (red) and electrode thickness (black). (c) Volume factions in the pristine and the lithiated electrode. The black values are gained based on the chemical composition of the electrode. The blue values are gained based on a segmentation of the tomographic data in the three phases. Reprinted with permission from Ref. [12• –14•• ]. Copyright 2015 Nature Publishing Group.; Copyright 2016 Nature Publishing Group.
Another leading edge of analyzing technique for volume expansion is operando tomography. Pietsch et al. reported microstructural changes of graphite and graphite–silicon blending electrodes (weight ratio of graphite and siliwww.sciencedirect.com
con = 75 (graphite): 25 (silicon) and active material ratio in electrode is 80 wt%) [14•• ]. In this work, the blending electrode showed expansion of 35% and reduction of the porosity by 8% (Figure 3C). This technique enables Current Opinion in Electrochemistry 2017, 6:77–83
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Figure 3
Analyzing results of SEI formation on Si-based anode (A) SEI compensation occurs during lithiation process (a) AFM images of SEI layers during (a–c) the first cycle, (c–e) the second cycle, and (e–g) the third cycle. (B) SEI growth (inorganic → organic) and SEI accumulation in pores, results of 13 C, 7 Li, and 19 F ssNMR with (a) 13 C EC (blue) and (b) 13 C DMC (purple) (C) electrochemically isolated Si nanoparticle by SEI formation, STEM-EELS element mapping of Si electrodes cycled at 30% of their theoretical capacity (1200 mAh/g) (a) after the first lithiation, unlithiated part covered by LiF, (b) after the first lithiation, cristalline Si/LixSi core-shell part covered by ∼20 nm thick carbonates layers and LiF particles. (c) After 100th delithiation, delithiated Si particles trapped in LiF matrix, (d) after 100th delithiation, delithiated Si particles covered by thick (∼100 nm) carbonate layer, (e) low-loss EEL spectra on 1 and 2 sites showing low lithium content on carbonate layer after extended cycling. Reprinted with permission from Ref. [17• –19•• ]. Copyright 2016 American Chemical Society.
to quantify the microstructural changes taking place in an electrode during lithiation/delithiation and it is expected to be useful for analyzing porous Si electrode.
Therefore, analyzing SEI layer as well as volume expansion is critical to establish the new strategies for Si-based anode materials.
SEI formation
Recently, one of the excellent approaches was focused on providing direct observation of SEI degradation by using in situ AFM [17• ]. In the work, the authors revealed significant difference between center and edge parts of Si active material, and SEI formation was preferred where extensive cracking occurs; that is the edge part. The crack formed during the first lithiation was not covered at low potential. At subsequent lithiation, the SEI layers on cracked site were developed (Figure 3A). Although the LIB academic circles have debated and predicted the place and timing on SEI formation for long time, it was mostly only prediction and indirect evidences.
Anode materials which have lower working potential than the reduction potential of carbonate electrolyte are not able to avoid formation of SEI layers [15]. Commercial graphite anode retains good cycling life in carbonate electrolyte, though SEI formation slightly reduces initial coulombic efficiency [16]. In contrast, Si anode suffers gradual capacity fading by continuous SEI formation during cycling because volume expansion brings new interface between Si and electrolyte [8]. Hence, combined effect of volume expansion and SEI formation; continuous SEI thickening causes electrochemically isolated Si part. Current Opinion in Electrochemistry 2017, 6:77–83
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Recent progress of analysis techniques for silicon-based anode of lithium ion batteries Son et al.
This work proved the actual and direct proof of SEI formation with experimental observation. On the other hand, material composition and distribution are additional inquisitive point in SEI studies. Michan et al. utilized 7 Li, 19 F, and 13 C solid state NMR (ssNMR), FIB, and SEM to investigate SEI growth and accumulation of SEI in the pores of electrode [18• ]. They compared the three electrodes at 1st, 30th, and 60th cycle. From the 19 F ssNMR, LiF peak intensity at 1st cycle was nearly maintained until 60th cycle, which means LiF species primarily formed at 1st cycle. On the contrary, 13 C ssNMR presented SEI including organic species such as –OCH2 CH2 O–, CH2 O, and functional groups CH3 R, RCH2 R, and CH3 CH2 R showing materials of ROCO2 Li, oligomeric species, Li ethylene carbonate, Li butylene dicarbonate, RCO2 Li, HCO2 Li, Li2 CO3 , and Li methyl carbonate. The signal intensities of these materials were increased after 30th cycle; it means organic SEI layers are grown. They also confirmed SEI formation induced the disappearing pore as cycle goes, by SEM images of FIB cross sections. Although NMR studies provided extensive view of the SEI components, it could not afford spatial information on particular particles. Boniface et al. demonstrated electron energy-loss spectroscopy (EELS) in a scanning trans-
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mission electron microscopy (STEM) mapping SEI components with resolutions down to 5 nm [19•• ]. Figure 3C shows the color mapping of SEI components on Si electrodes cycled at 30% of state of charge (SOC). In electrodes after both 1st and 100th cycle, LiF and carbonate components were detected and the thickness of carbonate layer was increased from 15 ∼ 25 nm up to ∼100 nm. They also revealed LiF-rich SEI layers was advantageous for lithiation compared to carbonate-rich SEI layers, which implies electrochemical isolation could be caused by carbonate buildups.
Phase transformation In precedent two sections, we focused on mechanical change (volume expansion) and chemical change (SEI formation) issues of Si-based electrode. From now on, this section would regard atomic change of Si-based electrode, in other words, phase transformation which could reflect kinetics and reversibility of Si and Lix Si. Formerly, structural change of the lithiation of Si was investigated by several methods such as in situ XRD, TEM, and NMR [10,20,21•• ]. From those, singular properties about phase transformation, such as amorphization of crystalline Si during lithiation, crystalline phase of Li15 Si4 , anisotropic lithiation into crystalline Si and etc., were introduced and studied so far. However, there have been only few outstanding outcomes in terms of sophisticatedly quan-
Figure 4
Analyzing results of phase transformation on Si-based anode (A) lithiation degree and crystallinity of Si (a) STEM-EELS Lix Si alloy composition mapping of Si electrodes after 10th lithiation to 1200 mAh/g (b) STEM-EELS Si crystallinity (plasmon peak position) mapping of Si electrodes (B) remained lithium in Lix Si after delithiation, the scattering length density profiles after (a) the second lithiation and (b) the second delithiation as function of time and distance from the interface (c-Si (green), the electrolyte (yellow), surface lithiation (red/dark-red), and deep lithiation (red-yellow)). Reprinted with permission from Ref. [19•• ,22•• ]. Copyright 2016 American Chemical Society. www.sciencedirect.com
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Batteries and Supercapacitors
titative and nanoscale-resolution analysis. The STEMEELS method which is introduced in previous section could be also proper technique to investigate phase transformation of Si electrode [19•• ]. With this technique, lithiation degree and crystallinity of Si can be observed; hence it could discover electrochemically isolated Si part by dense SEI layers (Figure 4A). Another technique is neutron reflectivity which enables non-destructive and quantitative analysis with nanometer-resolution scale. Seidlhofer et al. demonstrated in situ neutron reflectivity to investigate the alloying of bulk crystalline silicon [22•• ]. They utilized 1 cmsized single crystal Si and observed extent of lithiation into Si layer depending on the depth and time. The Li silicide layer was subdivided into two distinct parts; highly lithiated skin region (x ≈ ≈ 2.5 in Lix Si) and partially lithiated growth region (x ≈ ≈ 0.1 in Lix Si) (Figure 4B). After fully delithiation, the Li content in skin region remained as x 1.1 in Lix Si. The thickness of skin layer maintained, however, one of the growth regions increased as cycle goes (Figure 4B). It means there are inactivated and irreversible region in thick Si anode.
Acknowledgments This work was supported by the IT R&D program of MOTIE/KEIT (Development of Li-rich Cathode and Carbon-free Anode Materials for High Capacity/High Rate Lithium Secondary Batteries, 10046306).
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: •
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Conclusion In conclusion, we reviewed the unique characterizations of Si-based anodes according to three aspects; 1) volume change extended to mechanical properties, 2) SEI formation related to surface reaction on Si electrode, and 3) phase transformation which is atomic rearrangement. For further studies, analysis of Si-based anode should be focused on evaluation of higher scales; cells and electrodes which include effects on materials, binder materials, conductive materials, current collector, and electrolyte rather than that of only materials, for instance, in situ SEM or TEM observing cross sectional electrode expansion rather than just particle expansion, or in situ chemical element analysis detecting the exhaustion of electrolyte as well as SEI formation. These fundamental understandings of phenomena on Sibased anode are wished to finally derive the solution enabling wide use of Si-based anode in commercial LIBs. In detail, intrinsic expansion of Si active material itself could not be dynamically controlled in atomic scale, whereas the expansion in electrode level would rather be considered. Hence, the bad effects from electrode expansion have been controlled by pore structure, buffer matrix or binder materials which inhibit pulverization and cracking. Although we have to admit that SEI formation is inevitable due to intrinsic reduction potentials, it is still necessary to develop effective electrolyte making robust SEI layer. In addition, atomic design of Si-based materials should provide faster kinetics and higher reversibility founded on phase transformation studies. We believe and hope this review would be a great help to researchers developing Si-based anode for LIBs. Current Opinion in Electrochemistry 2017, 6:77–83
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