Silicon Anode Design for Lithium-Ion Batteries: Progress and

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Silicon Anode Design for Lithium-Ion Batteries: Progress and Perspectives Alba Franco Gonzalez, Nai-Hsuan Yang, and Ru-Shi Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07793 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 19, 2017

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The Journal of Physical Chemistry

Silicon Anode Design for Lithium-Ion Batteries: Progress and Perspectives Alba Franco Gonzalez,‡a,b Nai-Hsuan Yang‡a and Ru-Shi Liu*a,c a

b

Department of Chemistry, National Taiwan University, Taipei 106, Taiwan.

School of Chemistry, King’s Buildings, The University of Edinburgh, Edinburgh EH9 3JJ, UK. c

Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 106, Taiwan. ‡ These authors contributed equally to this work.

ACS Paragon Plus Environment

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ABSTRACT

Silicon has long been regarded as a prospective anode material for lithium-ion batteries. However, its huge volumetric changes during cycling is major obstacle to its commercialization, as these changes result in irreversible cracking and disconnection of the active mass from the current collector, as well as an excessive formation of a highly resistive solid electrolyte interphase.

Multiple

mechanical

stress

relief

strategies

that

primarily

use

silicon

nanostructurization have been previously developed. However, despite the significant improvements on the active material cycle life, using nanomaterials still result in complications, such as low conductivity, reduced volumetric energy density, and increased side reactions. This work provides a historical context for the development of silicon anodes and focuses on the surface chemistry and structural integrity of the electrode, thereby highlighting the most effective strategies reported recently for their optimization.


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1. Introduction Improved energy storage systems are necessary to meet the increasing global energy demand, which fluctuates daily and seasonally.1–3 Furthermore, the broadened environmental awareness has popularized electric vehicles and expanded renewable energies, which are limited by the intermittent energy production of their sources.2 Finally, the development of portable electronic devices and the increased dependency on these gadgets have reinforced the need for improved energy and power densities without compromising safety and costs.2–6 Rechargeable lithium-ion batteries (LIBs) have been widely used in portable electronics in the past two decades, and are promising candidates for fulfilling the requirements for future renewable energy storage grids, as well as for replacing petroleum fuels for automotive applications.2,3,7–11 At present, the storage capacity of commercial LIBs is limited by electrode materials.8 In particular, the maximum theoretical gravimetric capacity of the conventional graphite anode is only 372 mAh g-1, which is a conservative figure compared with 3800 mAh g-1 obtained from using lithium metal as the anode.11–16 Safety concerns regarding dendrite growth over the chargedischarge cycles make lithium metal anodes an infeasible alternative for rechargeable batteries. Nevertheless, replacing the anode with a material of improved theoretical capacity is still needed.8,15–17 LIBs function via a “Rocking Chair” mechanism.18 During the charge cycle, the graphite electrode is reduced and lithium ions intercalate reversibly between the graphite layers in a ratio of one lithium per six carbons, as represented by eq 1: Li + 6C LiC6

(1)

The opposite processes occur spontaneously during discharge, and the stored energy is released by lithium de-intercalation and graphite oxidation. The anode is the electrode where


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oxidation occurs and the cathode is where reduction occurs. However, these definitions would cause the electrodes to alternate names during the charge and discharge cycles, and thus for practical purposes, the electrodes are conventionally labeled with their role during the discharge cycle. To achieve increased energy densities, the intercalation chemistry of graphite must be replaced by anode materials that are capable of electrochemically alloying with lithium.9,19 Among these materials, silicon has the highest gravimetric capacity of 4200 mAh g-1, which corresponds to the formation of the amorphous Li22Si5, as shown in eq 2.5,20 However, X-ray diffraction studies by Obrovac and Christensen21 suggested that the reaction at room temperature undergoes a lower level of lithiation, corresponding to eq 3 and to a theoretical capacity of 3579 mAh g-1.11,22 This biphasic lithiation has since been confirmed by multiple in situ transition electron microscopy (TEM) studies.23,24 22Li + 5Si Li22Si5

(2)

15Li + 4Si Li15Si4

(3)

Silicon possesses other desirable qualities, such as being the second most abundant element in the Earth’s crust, cost-effective, and environmentally benign.25 Additionally, silicon has a low electrochemical potential between 0.37—0.45 V versus Li/Li+, which is 0.27 V higher than graphite but still provides an ample cell voltage on coupling with a cathode.19,26,27 Despite multiple advantages, the commercialization of silicon-based anode LIBs is being hindered by its poor cycling performance, wherein the failure mechanism is largely attributed to the severe volumetric expansion (~300%—400%) of this material that occurs during lithium insertion.4,7,8,17 As a result, the mechanical strain within the active mass increases, thereby leading to the


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cracking and pulverization of silicon and disconnection from the current collector, as displayed in Figure 1.1,28,29

Figure 1. Schematic of morphological changes occurring in bulk silicon upon electrochemical cycling. Enormous volumetric changes cause high mechanical strain, which in turn leads to cracking and disconnection from the current collector. In addition, the interphase between the anode and electrolyte is critically affected by volume changes.7 The low working potential of the anode, which is comparable to that of lithium ion reduction, lies outside the window of stability of most common electrolytes.30 During the first charge cycle or silicon lithiation, the reduction of both salts and solvents of the electrolyte at the silicon surface produces a solid layer called solid electrolyte interphase (SEI).18,31 This film may enhance battery performance by passivating the anode reactivity and preventing further electrolyte degradation, as is the case with conventional graphite LIBs.30,32 However, given the volume variation exhibited by silicon, the SEI layer often becomes deformed or breaks, exposing a fresh electrode surface where more electrolyte may be reduced during the succeeding charge cycle, as illustrated in Figure 2.6 This recurrent process consumes lithium and leads to the excessive growth of a thick, inhomogeneous, ionically insulating SEI; ultimately resulting in the battery’s capacity fading.7,30,33


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Figure 2. Schematic of SEI formation occurring at the surface of silicon upon electrochemical cycling. A thin SEI forms upon charging on the expanded state of lithiated silicon. During discharge, silicon’s volume decreases, forcing the SEI to break into separate pieces, and exposing fresh silicon surface to the electrolyte. More SEI is then formed on the following charge cycles and this recurrent process eventually leads to an excessively thick SEI surrounding the electrode.

Both mechanical and chemical degradations, along with the intrinsically low conductivity of silicon, comprise the key issues that lead to the poor cycling performance of this anode, and hamper its commercialization as a new-generation LIB. Subsequently, advances have been reported and follow three main strategies, namely, improving the mechanical properties of the electrode, enhancing the chemical stability of the electrode-electrolyte interphase, and increasing its conductivity. This review will illustrate the main approaches in literature and bridge the gap between structural and chemical knowledge.

2. Strategies for enhanced performance This section highlights the most heavily investigated methods for enhancing the performance of silicon anodes in Li-ion batteries. We grouped these strategies into six categories as listed in


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Figure 3: nanostructures, porous structures, unusual designs, composites, SEI formation, and binders.

Figure 3. Summary of the six most prominent strategies investigated to enhance the performance of silicon anodes for Li-ion batteries.

2.1 Nanostructures The mechanical strain relief attained by using nanostructures marked the largest breakthrough in the cycle life of silicon anodes. Particles under a critical size can withstand large volume variations without cracking.17,34 More specifically, the first in situ TEM study on silicon cells conducted by Liu and Huang23 determined the critical diameter size for the prevention of nanoparticle fracture to be 150 nm, as illustrated in Figure 4. Hence, silicon nanostructures with multiple morphologies have been designed, including nanopowders, nanotubes, nanofibers, nanorods, nanowires, and other nanoporous structures.11,35 However, the large surface area of these structures increases side reactions and further stimulates overgrowth of unnecessary SEI, thereby increasing the impedance for Li-ion transport and decreasing the capacity over cycling.30


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Figure 4. Size dependent fracture of Si nanoparticles during lithiation. (a) Pristine Si nanoparticles with diameters of 80 nm and 150 nm. (b–d) Lithiation process of the particles. The two phases (crystalline and amorphous) are clearly distinct. (e) The larger particle cracked while the smaller remained integral upon full lithiation. Reprinted with permission from ref. 23, Copyright 2011 The Royal Society of Chemistry.

Silicon nanowires were first synthesized by Chan et al.29 motivated by previous onedimensional studies of alternative anode materials. The nanowire anode reported by this group displayed astonishing capacities above 3000 mAh g-1, yet this capacity was only maintained for 10 cycles at the current rate of 210 mA g-1. There has been little improvement in the field since. In a recent study by Ramesh and Nagaraja36 nanowire anodes were cycled at a substantially larger rate of 4200 mA g-1 retaining a capacity of 1134 mAh g-1, but dropped to near zero values after the 12th cycle. It is speculated that the failure of nanowires arises from the earlier mentioned excessive SEI formation, internal mechanical stress, or from the facile delamination of the small point of contact of nanowires with the current collector.37,38 Analysis of the plausible failures of silicon nanowires lead Quiroga-González et al.38 to optimize the wire size and geometry, presenting wires with a controversially large diameter of 1


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µm. Until then, the upper limit for wire diameters was deemed to be 300 nm, yet the novel microwire anode endured a capacity of 3150 mAh g-1 for 100 cycles with a current rate of 2100 mA g-1.38 Concurrently, nanowire research inspired the synthesis of nanotubes by Park et al.39 This first nanotube design integrated carbon as a protective coating, which hampers its direct correlation to the performance of nanowires. A subsequent study by Wu et al.6 compared the performance of double-walled silicon nanotubes (DWSiNTs) to that of nanowires and nanotubes in the absence of coatings or conductive additives. The galvanostatic cycling curves are presented in Figure 5, where the capacity of each nanostructure is plotted as a function of the cycle number. The capacities retained by these nanotubes are slightly lower than that obtained through an independent investigation of bare silicon nanotubes by Song et al.37 which retained a capacity of 2000 by the 50th cycle under the same current rate. However, the graph still effectively highlights the difference in the capacities retained by nanowires, nanotubes, and DWSiNTs. It is worth noting that the high performance of DWSiNTs is attributed to the protective SiO2 outer layer, which buffers the excessive production of the SEI.6


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Figure 5. Galvanostatic cycling of double-walled silicon nanotubes, silicon nanotubes and silicon nanowires between 1 V and 0.01 V under a current rate of 840 mA g-1. Reprinted with permission from ref. 6, Copyright 2012 Macmillan Publishers Limited.

Thin film anodes are reported with very stable cycling and high capacities. Takamura et al.40 communicated the synthesis of a vacuum deposited silicon thin film which endured 1000 cycles with a reversible capacity above 2000 mAh g-1 at the vast current rate of 42000 mA g-1. However, to prevent irreversible morphological changes such as cracking, the anode size must be limited. The critical thickness is estimated to be between 100—200 nm for amorphous Si in a stainless steel substrate.37,41 This size constraint compromises the maximum capacity of the anode, so the long cycling with negligible fading of thin films cannot be exploited for the creation of viable batteries.37

2.2 Porous structures Porous structures work on the same principle as nanostructures by providing space for the volume expansion of silicon, thereby relieving mechanical strain. Porous structures are also particularly effective in shortening the diffusion pathways for Li-ions. A pioneer process for microporous silicon synthesis involved the magnesiothermic reduction of silica, as displayed in


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eq 4, followed by acid etching for magnesia removal. This method was first reported by Bao et al.42 and was later replicated by Liu et al.43, who exploited agricultural byproducts as the source of silica. 2Mg(g) + SiO2(s) 2MgO(s) + Si(s)

(4)

Similarly, several other synthesis methods in literature consist of two-step processes. In particular, Kim et al.44 used thermal annealing of silica followed by etching; Bang et al.45 employed silver deposition on silicon, followed by metal-assisted chemical etching; and Ge et al.46 obtained the porosity by ball-milling metallurgical silicon (high in Fe and Al impurities) with subsequent stain etching. Despite novel synthesis methods that provide the possibility of tuning particle and pore sizes, using porous silicon structures on their own seemed to have reached their electrochemical performance limits. To demonstrate this limitation, porous silicon obtained from rice husks by Liu et al.43 displayed a reversible capacity of 2650 mAh g-1 after 200 cycles at a rate of 2.1 A g-1. Another drawback to the use of either porous or nanomaterials is that their density is lower than that of raw silicon, thus, an increased cycle life is attained on account of volumetric capacity. Nonetheless, the majority of the investigations to date have exploited the long cycle life of a porous or nanosized active mass, complementing it with an experimental coating, binder or other innovative techniques.47

2.3 Unusual designs Several ingenious architectures have achieved exceptional results, such as the porous graphitic scaffold in Figure 6 designed by Zhao et al.48, which maintained a capacity of 2656 mAh g-1 after 150 cycles at a current rate of 1 A g-1. This particular structure showed good performance under


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high current densities and retained a capacity of 600 mAh g-1 at a rate of 4 A g-1 for the same number of cycles.

Figure 6. Schematic drawing of a section of a composite electrode material constructed with a graphenic scaffold with in-plane carbon vacancy defects. Silicon nanoparticles (large spheres) are enclosed between graphene sheets with in-plane vacancy defects, through which Li ions (small spheres) may readily diffuse. Reprinted with permission from ref. 48, Copyright 2011 Wiley.

Another notable framework was designed using silicon nanoparticles with carbon coating. Silicon coating has been proposed to enhance conductivity and reduce side reactions by forming a stable SEI. The pomegranate-inspired example by Liu et al.49 includes a void space between the silicon nanoparticle and carbon coating to accommodate the large volume of lithiated silicon particles. The coated silicon particles are clustered and coated with a thick layer of carbon, as displayed in Figure 7, to enhance conductivity and protection towards the electrolyte. The cells retained a considerable capacity of 1160 mAh g-1 for 1000 cycles at a rate of 635 mA g-1.


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Figure 7. Schematic of a pomegranate-inspired design. (a) 3D view and (b) simplified crosssection view of one pomegranate microparticle before and after electrochemical cycling. Reprinted with permission from ref. 49, Copyright 2014 Macmillan Publishers Limited.

The void space design is becoming common, and recent studies still use this advantageous feature in developing innovative structures, as did Zhu et al.7 for their flexible silicon and graphene/carbon nanofibers. Zhu et al.7 obtained 2002 mAh g-1 for 1050 cycles at a current density of 700 mA g-1.

2.4 Composites The presence of metals may increase the low conductivity of silicon and provide a range of mechanical and chemical properties.50 As a result, many studies have focused on intermetallic composites involving silver, magnesium, calcium, nickel, iron, cobalt, aluminum, copper, tin, and titanium, as well as some oxides of these metals.8,50–53 The increase in electrical conductivity is most often the aim of these studies, as composite anodes endure high current rates with a


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stable capacity retention. For instance, the tin doped silicon nanowires designed by Korgel et al.53 maintained a capacity above 1000 mAh g-1 for 100 cycles at the current rate of 2.8 A g-1. The significance of this investigation will become evident throughout this review, owing to rates above 1.5 A g-1 being seldom tested on silicon anodes. The combination of silicon with carbon is also popular due to its softness, high conductivity, and minimal volumetric change upon lithiation, thereby demonstrating competitive performance, as illustrated by the graphene/silicon nanosized sandwich structure designed by our group.54 However, the individual performance of reported composites is beyond the scope of this review. The detailed information on siliconbased composites may be found in the comprehensive reviews in Ref. 50–52 and the references therein.

2.5 Solid electrolyte interphase formation Some coatings are designed to recreate naturally formed SEI, albeit tuning the composition to minimize electrolyte reduction and enhance electrochemical performance, as illustrated in Figure 8. Commonly referred to as an artificially engineered SEI, this result is not exclusive to coatings but may also be attained by adding appropriate solvents, cosolvents, and electrolyte additives.15,35,55 A closely related strategy that may be conducted in parallel is using prelithiation, as demonstrated by Zhao et al.10, who combined prelithiated silicon with 1-fluorodecane to produce the artificial SEI. Generally, prelithiation involves the addition of either lithium or a lithiated compound to the anode to compensate for the irreversible lithium loss during SEI formation.10 The technique is also valuable as it facilitates the combination of high capacity anodes with lower capacity cathodes. This would otherwise require large quantities of cathode to


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be used in compensation for their lower capacity, or a restriction in mass, and thus in the capacity, of the anode.56

Figure 8. Schematic of SEI formation occurring upon electrochemical cycling of silicon coated with an artificially engineered SEI. A thin and stable SEI forms during the beginning cycles and passivates the reactivity of silicon towards electrolyte reduction.

2.6 Binders Given the popularity of nanomaterials for high-capacity anode applications, binders had to be developed to hold the active material and conductive particles together onto the current collector, as portrayed in Figure 9.9,30,33 Their enhanced mechanical properties suppress volume expansion and produce a stable SEI.33 Other advantages include: (1) the ease of synthesis of binder anodes, which require a mere physical grinding in the presence of a solvent, as opposed to the stringent chemical methods and vacuum conditions commonly used for the synthesis of nanowires, nanotubes, and porous structures; (2) the low-cost of reagents and anode processing; and (3) the possibility of using biodegradable/recycled polymers for environmentally friendly purposes. These interests, coupled with the competitive electrochemical performances obtained, locate binder anodes among top candidates for viable silicon based LIBs.


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Figure 9. Schematic of changes occurring in silicon anodes upon electrochemical cycling. (a) The use of nanoparticles alleviates mechanical strain and active mass can endure more cycles with little cracking. (b) The use of binders aid to maintain the structural integrity of the anode made of particles.

The focus of binder research seems to be on obtaining the best mechanical properties of the electrode as a whole, as much as the investigations for the design of artificial SEIs seem to be limited to surface chemistry. For this reason, and to integrate structural and chemical knowledge, the performances of both techniques are reviewed below. Although unrelated to previous categories, another notable strategy is the tuning of the electrochemical protocols to prevent anode deterioration. A study reported by Markevich et al.55 limited the charge or discharge capacity on amorphous silicon to reduce both the volumetric change and the electrolyte reduction. The theoretical rationale was successful and the group obtained an extraordinary cycling life of more than 2000 cycles at a current density of 600 mA g1

, though this was achieved by restricting the capacity to 600 mAh g-1. Even though the study

provided useful information about the chemical structure of a stable SEI, limiting the capacity or voltage window has direct repercussions on the charge and power stored. Therefore, this strategy


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is not as desirable as those without such proportionate effect. Moreover, the use of nanostructures, porous structures, composites, coatings, and binders all reduce the mass and volume percentage of the active content of the electrode, but this compromise on gravimetric and volumetric capacity is compensated by the prolonged cycle life and increased capacity retention achieved in the long run, which are highly desirable for commercialization.

3. The solid electrolyte interphase The SEI is produced by competing and parallel solvent and salt reduction processes, as indicated by the deposition of both inorganic and organic products on the electrode surface. Thus, it would be reasonable to assume that the composition of the SEI is directly related to the composition of the electrolyte.18 However, SEI formation is greatly affected by numerous variables, such as the dependency on the anode and cathode materials, the electrochemical method of formation, the presence/absence of binder and/or conductive additives, and the ambient conditions.18,30,31 These variables and inconsistencies among previous studies have produced results that vary widely from one research group to another.18,31 Despite the difficulties, understanding the morphology, chemical composition, and mechanism of SEI formation in relation to the variables, as well as the influence of these variables on battery performance, is essential for rational choices in future investigations.30 Just as most knowledge regarding silicon anodes, research towards the attainment of a stable SEI was inherited from previous investigations performed in carbon anodes. Electrolyte additives that aid produce a favorable SEI have been researched since the early 1990s. Those include inorganic additives such as CO2, N2O, CO, CO-complexes; crown ethers; isocyanate compounds; and vinylene groups or organic compounds capable of polymerization.57–64 Sulfur containing


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structures such as ethylene sulfite and polysulfides became reasonably popular, but even more so did fluorinated species, wherein research ranged from addition of HF to the use surfacefluorinated graphite.59,65–68 Fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are often included in silicon anode investigations as electrolyte additives or co-solvents, considering that their presence enhance cycle performance and thermal stability.35,69 An early study by Etacheri et al.15 related the superior performance of silicon nanowire anodes to the chemical composition and morphology of the SEI formed under the presence of those compounds. The FEC-containing cells retained a higher capacity than the FEC-free cells after 30 cycles, attributed to a thin and highly homogeneous SEI with a low ionic impedance and particular chemical composition. The major component of the SEI in FEC-containing cells was a polycarbonate that was formed from the polymerization of VC after this was produced from FEC, as suggested in Scheme 1. Competing reactions were observed in the presence of the solvent ethylene carbonate (EC), where its reduction formed a thick and less passivating SEI that is rich in oxygen species, including lithium carbonate and lithium ethylene dicarbonate (LEDC).15 The absence of highly reactive EC also accounted for a highly pronounced reduction of PF6- into fluorine-containing species, such as LiF.15


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Scheme 1. Suggested mechanism of formation of polycarbonate from FEC. Adapted with permission from ref. 15, Copyright 2011 American Chemical Society.

Nie et al.69 obtained similar results two years later in a study of SEI in FEC and EC-based electrolytes with binder-free (BF) silicon nanoparticle anodes. The absence of a binder limited the galvanostatic testing to 20 cycles; and although the FEC-based samples did improve capacity and structure retention, the electrochemical performance was not the focus of the study. A combination of analytical techniques was used to characterize the SEI composition and morphology. The film formed in the presence of FEC was thin and the predominant species in it were insoluble polymeric species (either polyene or polycarbonate), LiF, and LixSiOy. The presence of EC was again linked to the poor passivation of the electrode and electrochemical performance, which are attributed to the thick SEI primarily composed of LiF, LEDC, LixSiOy, and Li2CO3. The morphological differences of the nanoparticles cycled by this group in the presence and absence of FEC are visible in the TEM images in Figures 10 and 11, respectively, accompanied by the elemental concentrations at the surface determined by EDX. The thinner SEI of the FEC containing sample is apparent from the first cycle, and following twenty cycles, the nanoparticles remain more evidently discrete units than those in the EC based electrolyte.


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Figure 10. TEM images of BF-Si electrodes cycled in 1.2M LiPF6/FEC: (a)Fresh nanosilicon, (b) nanosilicon after 1st cycle, (c) nanosilicon after 5th cycle, and (d) nanosilicon after 20th cycle. Reprinted with permission from ref. 69, Copyright 2013 American Chemical Society.


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Figure 11. TEM images of BF-Si electrodes cycled in 1.2M LiPF6/EC: (a)Fresh nanosilicon, (b) nanosilicon after 1st cycle, (c) nanosilicon after 5th cycle, and (d) nanosilicon after 20th cycle. Reprinted with permission from ref. 69, Copyright 2013 American Chemical Society.

Schroder et al.30 established a dependency between electrochemical conditions and the formation of SEI, thereby obtaining high atomic percentages of alkyl species, ethers, and alkoxides by linear sweep voltammetry and cyclic voltammetry, as well as a high percentage of carbonates by chronoamperometry. Furthermore, this group proposed the mechanisms in Schemes 2 and 3 for the reduction of EC and diethyl carbonate (DEC) into LEDC, lithium alkyls, and alkoxides. This paper does not associate the battery performance to the SEI composition formed with different electrochemical protocols.


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Scheme 2. Suggested mechanism of formation of lithium alkoxide and alkyllithium from DEC. Adapted with permission from ref. 30, Copyright 2012 American Chemical Society.

Scheme 3. Suggested mechanism of formation of LEDC from EC. Adapted with permission from ref. 30, Copyright 2012 American Chemical Society.

A link of this sort was only provided by Markevich et al.55, who investigated the charge/discharge-restricted cycling mentioned in Section 2. The aim of the capacity restriction was to reduce volume variation and electrode degradation. However, SEIs that formed at different potentials had different compositions and passivation abilities. Moreover, the test involved the presence and absence of FEC and concluded that performance is both dependent on electrolyte composition and cycling protocol. The FEC-containing charge-limited cells were the


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only ones to retain the restricted capacity of 600 mAh g-1 for 2000 cycles. This SEI was thin and had polyenes and LiF as its major components. The discharge-limited cells and particularly those cells without FEC had the worst electrochemical performance, and their SEI had increased oxygen containing species. The presence of a polyene rather than a polycarbonate in this investigation was suggested to have resulted via two possible four-step mechanisms displayed in Scheme 4.

Scheme 4. Suggested mechanism of formation of polyene from FEC. Adapted with permission from ref. 55, Copyright 2013 The Electrochemical Society. Liu et al.32 attributed the disparities in the results to the analytical procedures used. The in situ study discredited other ex situ approaches due to the risk of mechanical or chemical changes occurring with solvent rinse, or reactions of the highly sensitive SEI with contamination, air, and humidity. Furthermore, the researchers suggested plausible components of the SEI, formed in a


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propylene carbonate (PC) solvent. These components are summarized in Table 1, together with the main compounds reported by various investigations and, if provided, an association to the influence of the presence of each compound on the passivating ability of the SEI.

Table 1. Compounds present in the SEI as reported in literature with relation to the solvent used and remarks on the electrochemical performance (*if provided). compound

solvent

ref.

remarks on performance

polycarbonate

FEC-DMC

15*

*enhancing

polyene

FEC-based

55*

*enhancing

polymer

FEC

31, 69*

*enhancing

LiF

EC, FEC, PC, EC-DEC

15*, 30, 31, 32, *enhancing 55*, 69*

organic

PC

32

-

alkyl

EC-DEC

30, 31

-

LiOR

EC, DMC, FEC, EC-DEC

15, 30, 31, 55*

*deteriorating

-RO2R, -RO2-

EC-DEC

30, 31

-

LixSiOy

EC or FEC

69*

*both

Li2CO3

EC-DMC, PC

15*, 32

*deteriorating

LEDC

EC, EC-DMC

15*, 69*

*deteriorating

carbonates

EC-DEC, PC

30, 32, 55*

*deteriorating

3.1 Artificial SEI Inspired by the reported SEI compositions that yielded superior performance, recent investigations have explored the use of coatings, such as LiF, long-chain organocarbonates, and even an unexpected yet promising combination of LiF with Li2CO3, considering that the last is


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usually associated to a poorly passivating SEI.10,15,70,71 Carbon, metal oxides, and doped silicon are other popular types of coatings designed to produce stable SEIs. Materials that are commonly used as electrolytes in solid-state lithium-ion batteries have been tested, as exemplified by lithium phosphorus oxynitride (LiPON).12–14,72–74 However, all investigations of artificial SEI lack standardization. As shown in Table 1, the use of different combinations of the common solvents FEC, EC, DEC, PC, dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC), drastically affected SEI composition. Moreover, different silicon nanostructures are often used, and in the case of nanoparticles (NPs), they are accompanied by different binders, which are discussed in the following section to have extremely disparate performances. The rate of the current used in charge and discharge has a non-linear effect on the capacity stored, and most investigations are performed under different current rates, thereby making their specific capacities incomparable to each other. Capacity retention is the specific capacity that the electrode is able to discharge after a specific number of cycles at a specific current rate. This current rate may be measured as a C rate, where 1C is a rate at which the electrode is completely discharged in 1 hour. Some groups considered the 1C rate to be 4.0—4.2 A g-1, which is close to the theoretical capacity of silicon, whereas others considered 1C to be 3.5—3.58 A g-1 for the maximum metastable alloying composition of Li15Si4 at room temperature.11,35,55,69,71 Therefore, it is more equitable to compare the performances in A g-1, but researchers often avoid disclosing what their 1C rate corresponds to. Despite the inconsistencies, various coatings for artificial SEI formation and their electrochemical performance values are summarized in Table 2 to gain perspective on the galvanostatic cycling quantitative limitations to date. The coulombic efficiency (CE) describes the stability of the anode capacity over cycling. CE is given as a percentage and calculated by


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dividing the charge exiting the anode during the delithiation of silicon over the charge entering during the lithiation process. The initial coulombic efficiency (ICE) is merely the CE for the first cycle, in which SEI formation accounts for the largest capacity loss. The rate capability is the capacity retained under high current densities, which is essential for commercial applications.


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Table 2. Reported electrochemical performance data for various artificial SEIs in silicon anodes. coating

LiPON

solvent

EC/DEC 3:7

capacity retention (mAh g-1)

current density

thin film 100 50 nm no binder

1600

N/A

N/A

N/A

75

NPs 70 50 nm PVDF

~1500

2.2

96.8

N/A

10

1200

0.2

52.2

~900 at 20

72

~3500

N/A

88.7 8

N/A

70

silicon and binder

cycles

1fluorode cane

EC/DEC

titanium oxide

EC/EMC NPs N/A 1:1 alginate

LiFLi2CO3

EC/DM C

1:1

1:2

100

thin film 50 ~100 nm no binder

ICE (%)

rate capability

ref.

(mAh g-1 at A g-1)

(A g-1)

nitrogen- EC/EMC porous doped PAA 3:7 carbon

100

1933

0.8

84

1904 at 20

74

nitrogen- EC/EMC NPs doped /DEC/FE

Silicon Anode Design for Lithium-Ion Batteries: Progress and

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