Abdelhamid, Muhammad, Snook, Graeme, Gaubicher, Joel, Lestriez, Bernard, Moreau, Philippe, Guyomard, Dominique, &O’Mullane, Anthony (2016)
Fabrication and performance of electrochemically grafted thiophene silicon nanoparticle anodes for Li-ion batteries.
Journal of Power Sources, 324, pp. 97-105.
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Fabrication and Performance of Electrochemically Grafted Thiophene Silicon Nanoparticle Anodes for Li-ion Batteries
Muhammad E. Abdelhamid, Graeme A. Snook * , Joel Gaubicher, Bernard Lestriez, Philippe
Moreau, Dominique Guyomard, Anthony P. O’Mullane *
Dr. M. E. Abdelhamid
School of Applied Sciences, RMIT University, GPO Box 2476V, Melbourne, VIC 3001, Australia
Dr. M. E. Abdelhamid, Dr. G. A. Snook
Mineral Resources, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Box 312, Clayton, VIC 3169, Australia
E-mail: [email protected]
Dr. J. Gaubicher, Dr. B. Lestriez, Dr P. Moreau, Dr. D. Guyomard
Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, 2 rue de la Houssinière, BP 32229, 44322 Nantes Cedex 3, France
A/Prof. A. P. O’Mullane
School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia
E-mail: [email protected]
Keywords: Si nanoparticles; grafting; thiophene; conducting polymer; Li-ion batteries; Si anodes
In this work, we developed thiophene-grafted Si nanoparticle anodes for Li-ion batteries. The surface of Si nanoparticles was chemically modified with thiophene molecules via an electrochemical grafting process. This study is an overture for future endeavours where the thiophene layer on the modified Si- nanoparticles will function as an anchor point when co-polymerised with conducting polymers to produce a chemically bonded Si/conducting polymer anode composite. This in principle should mitigate the loss of electrical connection with Si due to any structural breakdown that may occur during cell cycling. Furthermore, the effect of the thiophene layer on the capacity and the cyclability of the Si electrode was apparent where the new material retained ~39% of the starting capacity after 60 cycles, compared to the unmodified Si electrodes, that merely lasted for 10 cycles with only 4% capacity retention. This is due to the thiophene layers which create an artificial solid electrolyte interphase around the Si nanoparticles protecting them from degradation.
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Additionally, the viability of using conducting polymers as anode composites under charging/discharging conditions was investigated via electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) to conclude whether or not n-doping of the conducting polymer is required in such applications.
1. Introduction
In a modern age characterised by the inevitable transformation from using fossil fuels to greener renewable energy sources, new cutting-edge materials for energy storage are being pursued by scientists to keep up with the surging demand for clean energy. Such materials should be able to store or generate high amounts of energy in devices that ultimately should be cheap, lightweight, environmentally friendly and easily produced to maximise cost efficiency. Traditionally, energy storage devices such as Li-ion batteries utilise graphite based materials as anodes, but a significant drawback is that graphite exhibits low capacity that can't match the full energy capacity of lithium [1-3]. In order to overcome this problem, other materials, such as silicon, are being utilised as anode materials instead of graphite. This is due to silicon’s extraordinarily high gravimetric capacity (3572 mAh g-1
vs. 372 mAh g-1 for graphite) as well as high volumetric capacity (8322 mAh L-1 vs. 818 mAh L-1 for graphite) [4, 5]
.However, silicon-based anodes have their own disadvantages due to their massive volume expansion that can reach up to ~400% during the lithiation process [6]. This massive volume expansion results in a structural fracture and hence, lose of electrical contact with the collector electrode [7] as well as breaking and reformation of an unstable solid electrolyte interphase (SEI) layer in the subsequent cycles which consumes the electrolyte [8]. All of this results in a large fade in capacity [4]. Additionally, the conductive mixture, which is required for the anode to work properly, adds extra weight to the battery without contributing to its capacity which is detrimental to its applicability in applications such as electric vehicles.
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Many strategies have been attempted to tackle these problems [1, 9]. Nano-sized silicon particles were utilised to better accommodate the colossal strain which mitigated the cracking issue [6, 10]. Furthermore, Si nanoparticles were mixed with various polymeric binders, such as polyacrylic acid (PAA), polyvinylidene difluoride (PVdF), and carboxyl-methyl cellulose (CMC), to provide strong bonding between the Si nanoparticles and the conductive carbon material in order to improve the cycle life [11, 12]. The cycle life of Si/PAA and Si/CMC composites anodes were improved due to the binding between the functional groups on the polymers and the oxide layer on the silicon particles [1]. Recently, various types of conducting polymers (CPs) were utilised with Si electrodes. An exotic variant of CPs was tested by Liu et
al. where the polyfluorene-type polymers served as a conductive binder to overcome the
volume expansion problem [12]. Furthermore, Cui et al. achieved a high-performance Li-ion battery by developing a silicon/polyaniline (PANi) hydrogel composite anode [2]. They encapsulated the Si nanoparticles within the PANi 3D porous network via in situ polymerisation. In both cases, the CPs served as conductive networks for fast electron and ion transport as well as providing the proper support for the Si to accommodate any volume expansion. However, for Li-ion batteries, the charging/discharging process on the anode takes place at a very negative potential (vs. Li/Li+) [3] in a region where the CPs are n-doped, as in PEDOT and polypyrrole, or undoped, as in the case of polyaniline. Due to this phenomenon, the use of CPs as battery electrode materials receives some criticism. That is because at this very negative potential range the conductivity of the CPs is thought to be at its lowest where the polymer structure lacks counter ions, as in polyaniline, or contains a very low concentration of counter ions, as in n-doped PEDOT. Furthermore, the n-doping process of PEDOT has been found to diminish and disappear in the presence of Li+ ions in the solution [13]
.
The aforementioned work on developing Si/CPs composite anodes relied on the physical incorporation of Si nanoparticles within the CPs. These methods do not provide strong
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chemical bonding between the CPs and the Si nanoparticles whether the composites were prepared via physical mixing of CPs and Si, or via in situ polymerisation of CPs with the nanoparticles. In the presented work, a new approach is taken, whereby the surface of Si nanoparticles was chemically modified with thiophene molecules via an electrochemical grafting process. In future studies, this thiophene layer is proposed to serve as anchoring points for poly(3,4-ethylenedioxythiophene) (PEDOT) to attach to during polymerisation in order to yield a chemically bonded Si/CPs composite anode. The relatively strong Si-C covalent bond [14] formed between the Si surface and the C atom from thiophene should ensure the lasting connection between the Si nanoparticles and the CP matrix and hence mitigate the effects of volume expansion exhibited by the Si electrodes. This study serves as a preface for these future studies and focuses on the electrochemical grafting process of thiophene onto silicon and what effect the thiophene layer, alone, has on the behaviour and performance of the modified Si electrodes and if it offers a viable route to protecting the Si nanoparticles against degradation during lithiation and de-lithiation processes Additionally, the study investigates the viability of using conducting polymers such as PEDOT as anode composites in Li-ion batteries via electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) to determine whether or not n-doping of the conducting polymer is required for such applications.
2. Experimental Section
Materials: 3,4-ethylenedioxythiophene (97%, EDOT), bromo-thiophene, acetonitrile (≥
99.0%), N-methyl-2-pyrrolidone (99%, NMP), Tetrabutylammonium hexafluorophosphate (NBu4PF6), hydrofluoric acid (HF), and carboxymethyl cellulose (CMC, Mw= 90000) were purchased from Sigma-Aldrich and 1 M Lithium hexafluorophosphate in ethylene carbonate and dimethyl carbonate (LP30 battery standard electrolyte, LiPF6 EC/DMC) was purchased from Solvionic. Tetraethylammonium tetrafluoroborate (NEt4BF4) was purchased from
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nanoparticles (~150 nm in diameter) were synthesised by CEA-LITEN (Grenoble, France). For the characterisation studies, a Kratos Nova XPS instrument, Bio-Logic battery testing unit,
FEI Nova SEM, and Hitachi HF-2000 Field Emission Gun (FEG) TEM were used.
HF treatment of Si nanoparticles: The Si nanoparticles (150 nm diameter) were treated with
HF to etch away the oxide layer from the Si surface to generate an H-terminated surface. The H-Si bond is crucial for the grafting of the thiophene layer onto the Si nanoparticles. First, 0.5 g (± 0.05) of Si nanoparticles were loaded into two centrifuge tubes and then 30 mL of 2% HF solution (in NMP) was added to each tube. Both tubes were then ultrasonicated for 30 minutes in order to suspend the nanoparticles in the HF solution and expose more surface area to the etching reaction. After the ultrasonication for 30 minutes, the nanoparticles suspension was vacuum filtered through a Teflon filter membrane and then washed with NMP for 5 times and an excessive amount of absolute ethanol until the pH of the filtrate was ~7.
Thiophene grafting: The electrochemical grafting was achieved via a modified version of the
Gurtner et al. method [17]. Following the etching, the Si nanoparticles were resuspended into 25 mL 50 mM bromo-thiophene in acetonitrile and 100 mM NEt4BF4 as supporting electrolyte. The new suspension was placed in an electrochemical cell and purged with argon for 30 minutes. The working and counter electrodes were a 2 cm2 stainless steel mesh and Ag/AgNO3 (10 mM AgNO3 + 100 mM NBu4PF6 in acetonitrile) was used as a reference electrode (Figure 1a).
After purging for 30 minutes, the electro-grafting was undertaken by galvanostatic methods where the current was held at -20 mA for 15 minutes and the suspension was kept under stirring during the process, this step was repeated twice with 5 minutes rest in between. The reaction mixture was left to stir for 5 minutes after the electro-grafting was finished. The product was then filtered through a Teflon membrane and washed several times with anhydrous acetonitrile. The Si nanoparticles were collected and examined for the formation of thiophene layer via X-ray photoelectron spectroscopy (XPS) and TEM (with EDAX).
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The weight ratio of thiophene and Si was hard to measure as thiophene was deposited as a grafted monolayer, hence, its weight is negligible. Measuring the weight ratio was not possible by way of subtracting the weight before and after the grafting process as the yield was not 100%. However, we estimated the ratio to be around 2.3% assuming one thiophene molecule per surface Si atom on an (110) plane. The EDAX data shows that the weight ratio of sulfur ranges from ~1.9 - 3.3%. This number is variable due to the limited depth that the beam penetrates but gives an indication of incorporation of a thiophene layer.
Electrode preparation and cell assembly: Li-ion battery anodes were made out of the
thiophene modified Si nanoparticles as well as untreated Si nanoparticles as a control sample. The Si or the Si-thiophene nanoparticles were mixed with CMC binder and carbon black with the ratio of 72% Si (Si-thiophene): 20% carbon black: 8% CMC. Typically, the CMC was weighed and dissolved in Milli-Q water and then stirred for 4 hours before adding the Si nanoparticles as well as the carbon black powder. After adding the Si and carbon materials into the CMC solution, the resultant slurry was stirred overnight and its viscosity was adjusted by adding a few microliters of water.
On the following day, the slurry was cast onto copper foil and spread via doctor-blading to 100 μm thickness and then left to dry in the fume hood overnight. The dried film was transferred into a vacuum oven at 100 ºC for 2 hours, then 10 mm diameter discs were punched from the film and transferred back into the oven for 1 hour. After drying the discs, the loading of the active materials per cm2 was determined to be 1.025 mg cm-2 for the Si-thiophene anode and 0.897 mg cm-2 for the unmodified Si electrodes.
The Si-thiophene and the untreated Si electrodes were transferred into the glove box for battery assembly. 12 mm discs were cut out of Li metal sheet and fixed onto copper discs with the same size then placed into Swagelok cells, which served as battery cells (Figure 1b). Then, the Li discs were covered by a double layer of glass fibres cell separator, followed by adding ~ 900 µL of 1 M LiPF6 in EC/DMC 1:1 to the cell separator. Finally, the Si electrode
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discs were added onto the previous assembly and the Swagelok cell was tightly closed and left to rest for 12 hours before any measurement. The cell was connected to a battery testing unit and the charge/discharge rate was C/7 (i.e. a full charge/discharge lasting 7 hours) where the current applied was determined by the mass loading of the anode.
Impedance measurements: PEDOT was electrochemically polymerised via immersing two
freshly cleaned stainless-steel meshes into a 0.1 M solution of EDOT in 0.1 M NEt4BF4/acetonitrile and then the potential was biased at 0.85 V vs. Ag/Ag+ for 15 min or until 5 coulombs of charge had passed. Any excess EDOT monomer as well as oligomers were removed by washing in acetonitrile several times. The electrochemical impedance spectroscopy (EIS) experiments were carried out on the electropolymerised PEDOT using a Bio-Logic multipotentiostat (VMP3) across a range of frequencies (1 mHz to 1 MHz) at different biased potentials in 1 M LiPF6 EC/DMC, using a Ag/Ag+ reference electrode (with 10 mM AgNO3 in 0.1 M NBu4PF6/Acetonitrile) and stainless-steel mesh as the counter electrode.
3. Results and discussion
The electrochemical grafting was achieved successfully when 2-bromothiophene was used as a precursor for thiophene in the electrochemical grafting process. When a negative current (i.e. -20 mA, Figure 2a) was applied, the 2-bromothiophene underwent an irreversible reduction process, as shown in Figure 2b, where reductive cleavage of the C-Br bond occurred on the stainless steel electrode following a dissociative electron transfer (DET) mechanism (Equation 1, 2) [15]. The reductive cleavage yielded the Br− anion as well as the thiophene radical [16].
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This is followed by the reaction of the produced thiophene radicals with the hydrogenated Si (Si-H, due to HF treatment) nanoparticles present in the solution as a suspension (equations 3, 4) [17]. The reaction occurs in two steps. In the first step, thiophene radicals attack the hydride Si surface and results in Si radicals and neutral thiophene. The second step involves further reaction of thiophene radicals with Si radicals on the surface of the nanoparticles.
Stirring the reaction solution/suspension ensures a homogenous medium and migration of new Si nanoparticles towards the electrode surface where the thiophene radicals are produced. Also, stirring aids the diffusion of the thiophene radicals away from the electrode surface and hence clearing the electrode surface for more 2-bromothiophene molecules to be reduced (i.e. rapidly increase the concentration of thiophene radicals in the reaction media). This is apparent in Figure 2a where the recorded potential due to the galvanostatic reduction of 2-bromothiophene is lower when the solution was stirred. Also, the current recorded during cycling the potential while stirring is about 3 times higher than for the non-stirred situation due to enhanced diffusion of fresh 2-bromothiophene molecules to the electrode surface (Figure 2b). The formation of a thiophene layer onto the silicon nanoparticles was confirmed by XPS spectroscopy where the sulphur peak for the 2p orbital at ~165 eV [18], which corresponds to thiophene, was apparent (Figure 3a, b). The sulphur peak can be attributed to the thiophene layer as it is the only species that contains sulphur. Also, the formation of
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polythiophene is discounted as the polymerisation of thiophene occurs via oxidative polymerisation at more positive potentials whereas the grafting process employed here occurs via reduction at negative potentials ensuring that only thiophene is formed on the surface of the Si nanoparticles [18]. Also, TEM micrographs show the difference between the edges of the untreated Si and the Si-thiophene nanoparticles (Figure 3c, d). In the case of the Si-thiophene particles, there is an amorphous edge surrounding the Si crystal which contrasts the well-defined sharp crystalline edge observed in the untreated silicon. This amorphous feature, less than 5 nm, is attributed to the thiophene layer which is confirmed by EDAX spectra (Figure S1) of the Si-thiophene nanoparticle shown in figure 3d.
In order to compare the performance of untreated and thiophene grafted Si nanoparticles anodes, cycle life electrochemical tests were carried out for 60 cycles. Figure 4a reveals the evolution of the charge and discharge capacities with cycling. The decay in the discharge capacity with cycling is significantly lower in the case of the thiophene grafted Si electrode being ~778 mAh g-1 after 60 cycles (i.e. ~39% of the initial capacity) while the untreated Si electrode merely survived 10 cycles and reached a capacity of ~61 mAh g-1 after 60 cycles (i.e. ~4% of the initial capacity). The coulombic efficiency (i.e. charge/discharge capacity ratio) of the thiophene grafted Si was ~84% for the first cycle and goes up to a steady state between 96.2 and 97.5% for the following cycles (Figure 4b). On the other hand, the initial coulombic efficiency of the untreated Si electrode was ~86% and then stabilizes above 100% for the rest of the cycles, which means the discharge values exceeded those of the charge. This is due to the rapid failure of the untreated Si electrode where the battery is not charging to its full capacity in each consecutive cycle.
Figure 5 shows the first cycle voltage curves and subsequent cycles, as well as the decay of capacity with cycling for the untreated Si and thiophene-grafted Si electrodes. The first cycle voltage curve of the untreated Si electrode (Figure 5a) exhibits a well-defined discharge plateau at ~0.1 V and during charging at ~0.45 V. In comparison, the first cycle voltage curve
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of the thiophene grafted Si electrode (Figure 5b) exhibits the same well-defined plateaus for discharging at ~0.1 V and in charging at ~ 0.45 V. These plateaus are usually an indication that the Si particle size is in the range of a few microns [3]. The stability of both composite anodes and their rate of degradation could be deduced from Figure 5 as well as Figure 6. The rapid capacity drop of the untreated Si electrode is obvious where it took ~250 hr to complete 60 cycles. On the other hand, at ~250 hr, the thiophene grafted Si electrode completed 9 cycles and completed the 60th cycle in ~1100 hr.
One can conclude from this data that the thiophene layer enhances the cycling life of the Si nanoparticles anodes as it prevents the capacity of the anode from degrading in the first 20 cycles, unlike the untreated Si electrode. Furthermore, the thiophene layer enhances the starting capacity of the Si-thiophene anode by ~500 mAh g-1. It is unlikely that the extra capacity is related to the thiophene layer as no extra electrochemical process was observed in the charge/discharge cycling graph (Figure 6). This suggests that the thiophene layer serves as an artificial solid electrolyte interphase (SEI) protecting the Si nanoparticles from electrolyte attack and subsequent breakdown of the electrolyte.
Given that a thiophene layer shows promise in stabilizing Si against degradation, the next focus of this work was to probe whether conversion of this layer into PEDOT would be, in fact, of benefit given the negative potentials employed in Li-ion batteries at which conducting polymers do not maintain their ionic conductivity. Figure 7a shows the cyclic voltammogram of the electrolpolymerised PEDOT (see Experimental Section for details) on stainless steel mesh in a Li+ ion containing electrolyte where the n-doping peak was dramatically inhibited and completely disappeared after only one cycle, in contrast to the situation when the electrolyte doesn’t contain Li+
ions(Figure S2). This phenomenon was originally reported by Levi et al. with polythiophene [13]. Moreover, the EIS data of the polymerized PEDOT film on the stainless steel electrode show the difference in the impedance value between the n-doping (0.1 V vs. Li/Li+) and p-doping (3.9 V vs. Li/Li+) regions (Figure 7b, c) where the ionic
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resistance is 421 Ω and 12 Ω for n- and p-doping regions respectively, which were confirmed by fitting the data to an equivalent circuit model (Figure 8a). The error values for the 5 elements equivalent circuit, which are listed in the caption of Figure 8, are an indication of the good fit of the equivalent circuit, however when a simple 3 elements circuit were used, the data didn’t fit properly. This equivalent circuit model consists of a resistance (R1) and two subcircuits connected in series, where R1 represents the resistance of the solution and the electronic resistance of the polymer combined. The first sub-circuit is attributed to the compact inner part of the PEDOT film and is composed of a constant phase element (CPE1) connected to a resistance (R2) in parallel. Here, CPE1 and R2 are attributed to the distributed capacitance and the ionic resistance, respectively, due to the migration of Li ions within the compact PEDOT film (Figure 8a) [19]. On the other hand, the second sub-circuit is attributed to the porous outer part of the PEDOT film (Figure 8a) where the polymer/solution interface exists. This sub-circuit is composed of another constant phase element (CPE2) and a Warburg element (Wo1). In this case, CPE2 represents the distributed capacitance of the polymer interface/double layer of PEDOT and the electrolyte solution, while Wo1 represents the ionic diffusion of Li+ ions into the bulk of the polymer [20]. Figures 7b and 7c show the Nyquist plots of the whole frequency domain where the frequency part of the plots shows a low-frequency capacitance of the n-doped PEDOT film to be 4.2 mF compared to 120 mF for the p-doped film. This data shows that inhibition of ion insertion into the conducting polymer is extremely high, which will restrict ion flow within the conducting polymer. Further examination of the EIS data is shown in Figure 8b and c, where the largely deformed semicircle of PEDOT varies depending on whether the conducting polymer is at a low potential (0.1 V vs Li/Li+) or p-doped at a higher potential (3.9 V vs Li/Li+).
Interestingly, when looking closely at the data, the PEDOT film exhibits similar electronic resistance (25.8 Ω and 20.4 Ω for n- and p-doped respectively) at the two different potential regions: p-doping region (3.9 V vs. Li/Li+) and n-doping region (0.1 V vs. Li/Li+). This
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suggests the de-doping process is not fully complete and that some ions remain in the polymer to allow for reasonable electronic conductivity despite the low potential applied to the conducting polymer. Consequently, if this material is used as a conducting filler or binder for silicon anodes, electrical current can still be conducted through the material while restricting its physical expansion due to ion ingress into the polymer. The use of conducting polymers in silicon anode systems has been criticised in the past due to irreversible capacity from the intrinsic capacitive properties of the conducting polymer itself. These findings suggest that in fact this material can be used efficiently and effectively as both a binder (with its flexible properties) and a conducting matrix (with its electrical conductivity) in such a technology.
3. Conclusion
The electrochemical grafting of thiophene onto Si nanoparticles was successfully achieved as demonstrated and proved by XPS spectra. The thiophene-grafted Si nanoparticles enhanced the Si electrode behaviour regarding the cyclability, stability, and the starting capacity. Batteries utilising the thiophene-grafted Si as anodes retained ~39% of their initial capacity and maintained a coulombic efficiency of ~97%. On the other hand, the untreated Si nanoparticles merely lasted for 10 cycles and the capacity dropped to only ~4% of the initial capacity after 60 cycles. Although the grafted Si nanoparticles were not meant to be utilised on their own as anode materials, the charging/discharging data is promising compared to untreated Si nanoparticles. This could be attributed to the formation of an artificial layer around the Si nanoparticles that acts like a solid electrolyte interphase (SEI) layer protecting the surface of the nanoparticles and consumption of the electrolyte.
The EIS study discussed in this paper also suggests that PEDOT still exhibits a relatively high electrical conductivity in its n-doped state or at low potentials (~ 0.1 V vs Li/Li+). The electronic conductivity is maintained even when the n-doping process is suppressed by the
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presence of Li+ ions in the electrolyte, even though the ionic conductivity drops significantly. Nevertheless, the absence of ionic intercalation within the polymer structure, due to the inhibition of the n-doping process, is favourable in this case as it prevents the polymer from breaking down mechanically due to swelling/de-swelling. Also, it eliminates the capacity loss associated with the polymer doping process. It is believed that this result provides an important insight into the application of PEDOT, as one of the promising conducting polymers, as an electrode material for Li-ion batteries and explains why polymers which cannot be n-doped, such as polyaniline have provided improved properties when incorporated into silicon anodes [2].
Finally, it is important to note that this study is a preface and would be extended to the use of the thiophene-grafted Si as a starting material for making PEDOT-grafted Si nanoparticles, via in situ polymerisation with EDOT. The PEDOT would provide a conducting binder matrix that is chemically bonded to the Si moiety of the anode. This in principle should mitigate the loss of electrical connection with silicon due to any structural breakdown that may occur during cell cycling. Also, according to previous reports, polymer encapsulation will exhibit a protective SEI effect on the nanoparticles. We think that the results described herein will pave the way to the next modification step to produce efficient Si/CPs composite anodes, and will have a great potential towards further enhancing the anode behaviour.
Acknowledgements
MEA acknowledges the CSIRO for the provision of a Ph.D. stipend and RMIT University for the provision of a publication writing grant. AOM gratefully acknowledges funding from the Australian Research Council through a Future Fellowship (FT110100760). GAS acknowledges the funding from the CSIRO Office of the Chief Executive Julius Career Award.
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[1] M. E. Abdelhamid, A. P. O'Mullane, G. A. Snook, RSC Adv. 2015, 5, 11611.
[2] H. Wu, G. Yu, L. Pan, N. Liu, M. T. McDowell, Z. Bao, Y. Cui, Nat. Commun. 2013,
4, 1943
[3] M. Gauthier, D. Mazouzi, D. Reyter, B. Lestriez, P. Moreau, D. Guyomard, L. Roue,
Energy Environ. Sc. 2013, 6, 2145.
[4] D. Mazouzi, N. Delpuech, Y. Oumellal, M. Gauthier, M. Cerbelaud, J. Gaubicher, N. Dupré, P. Moreau, D. Guyomard, L. Roué, B. Lestriez, J. Power Sources 2012, 220, 180. [5] M. N. Obrovac, L. Christensen, D. B. Le, J. R. Dahn, J. Electrochem. Soc. 2007, 154, A849.
[6] U. Kasavajjula, C. Wang, A. J. Appleby, J. Power Sources 2007, 163, 1003.
[7] J. H. Ryu, J. W. Kim, Y.-E. Sung, S. M. Oh, Electrochem. Solid-State Lett. 2004, 7, A306; L. Y. Beaulieu, K. W. Eberman, R. L. Turner, L. J. Krause, J. R. Dahn, Electrochem.
Solid-State Lett 2001, 4, A137.
[8] C. C. Nguyen, S.-W. Song, Electrochim. Acta 2010, 55, 3026; L. Baggetto, R. A. H. Niessen, P. H. L. Notten, Electrochim. Acta 2009, 54, 5937; M. Winter, Zeit.Physik. Chem. 2009, 223, 1395.
[9] W.-J. Zhang, J. Power Sources 2011, 196, 13.
[10] H. Kim, M. Seo, M.-H. Park, J. Cho, Angew. Chem., Int. Ed. 2010, 49, 2146; Y. Yao, M. T. McDowell, I. Ryu, H. Wu, N. Liu, L. Hu, W. D. Nix, Y. Cui, Nano Lett. 2011, 11, 2949; C. K. Chan, H. Peng, G. Liu, K. McIlwrath, X. F. Zhang, R. A. Huggins, Y. Cui, Nat.
Nanotechnol. 2008, 3, 31; L.-F. Cui, Y. Yang, C.-M. Hsu, Y. Cui, Nano Lett. 2009, 9, 3370;
M. N. Obrovac, L. J. Krause, J. Electrochem. Soc. 2007, 154, A103.
[11] I. Kovalenko, B. Zdyrko, A. Magasinski, B. Hertzberg, Z. Milicev, R. Burtovyy, I. Luzinov, G. Yushin, Science 2011, 334, 75; A. Magasinski, B. Zdyrko, I. Kovalenko, B.
15
Hertzberg, R. Burtovyy, C. F. Huebner, T. F. Fuller, I. Luzinov, G. Yushin, ACS Appl. Mater.
Interfaces 2010, 2, 3004.
[12] G. Liu, S. Xun, N. Vukmirovic, X. Song, P. Olalde-Velasco, H. Zheng, V. S. Battaglia, L. Wang, W. Yang, Adv. Mater. 2011, 23, 4679.
[13] M. D. Levi, Y. Gofer, D. Aurbach, Polym. Adv. Technol. 2002, 13, 697. [14] Handbook of Chemistry and Physics, CRC Press, 2015.
[15] S. Arnaboldi, A. Bonetti, E. Giussani, P. R. Mussini, T. Benincori, S. Rizzo, A. A. Isse, A. Gennaro, Electrochem. Commun. 2014, 38, 100.
[16] A. A. Isse, P. R. Mussini, A. Gennaro, J. Phys. Chem. C 2009, 113, 14983. [17] C. Gurtner, A. W. Wun, M. J. Sailor, Angew. Chem. Int. Ed. 1999, 38, 1966. [18] M. E. Abdelhamid, G. A. Snook, A. P. O'Mullane, ChemPlusChem 2015, 80, 74. [18] A. Zaban, D. Aurbach, J. Power Sources 1995, 54, 289; D. Aurbach, A. Zaban, J.
Electrochem. Soc. 1994, 141, 1808.
[19] C. R. Yang, J. Y. Song, Y. Y. Wang, C. C. Wan, J. Appl. Electrochem. 2000, 30, 29; G. A. Snook, A. S. Best, J. Mater. Chem. 2009, 19, 4248; S. M. Richardson-Burns, J. L. Hendricks, B. Foster, L. K. Povlich, D.-H. Kim, D. C. Martin, Biomaterials 2007, 28, 1539; A. Ramanavicius, A. Finkelsteinas, H. Cesiulis, A. Ramanaviciene, Bioelectrochemistry 2010,
79, 11; J. F. Rubinson, Y. P. Kayinamura, Chem. Soc. Rev. 2009, 38, 3339.
Figure Captions
Figure 1. (a) cell setup for electrochemical grafting of thiophene onto Si nanoparticles using 2-bromothiophene as a precursor. (b) illustration of the Swagelok cell assembly.
Figure 2. (a) Potential response on stainless steel mesh electrode during galvanostatic grafting of thiophene on silicon nanoparticles from 50 mM 2-bromothiophene in 100 mM NEt4BF4/acetonitrile at -20 mA for 15 minutes. (b) cyclic voltammogram recorded at a stainless steel electrode in the same electrolyte showing the irreversible reduction of 2-bromothiophene. The figures show the response of the potential in relation to the agitation state of the suspension (i.e. stirring and no stirring).
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Figure 3. Graphs (a) XPS survey spectra of thiophene grafted Si nanoparticles showing the S, Si, and O peaks, (b) spectra S2p peak that confirms the presence of a thiophene layer on the silicon nanoparticles. (c, and d) TEM micrographs for untreated Si and Si-thiophene nanoparticles, respectively. The micrographs shoe the crystalline metallic edge of the untreated Si and the amorphous layer covering the crystalline Si in the Si-thiophene nanoparticles.
Figure 4. Graphs show the cycling performance for the untreated Si and thiophene grafted Si electrodes; (a) the charge/discharge capacity and (b) the efficiency as a function of the number of cycles.
Figure 5. Graphs show the galvanostatic cycles for (a) the untreated Si, and (b) the Si- thiophene anodes. The first cycle is in black in both graphs.
Figure 6. Charge/discharge curves showing the periodic change in voltage as a function of time for (a) untreated Si, and (b) Si-thiophene anodes.
Figure 7. (a) Cyclic voltammetry curve recorded at 20 mV s-1 of PEDOT (p- to n-doping region) showing the inhibition of the n-doping of the polymer in the presence of Li+ ions in the solution (1 M LiPF6 EC/DMC). Nyquist plots of; (b) n-doped (at 0.1 V vs. Li/Li+) and (c) p-doped (at 3.9 V vs. Li/Li+) PEDOT in 1 M LiPF6 EC/DMC, using a Ag/Ag+ reference electrode (with 10 mM AgNO3 in 0.1 M NBu4PF6 in Acetonitrile) and stainless-steel mesh as the counter electrode across a range of frequencies (1 mHz to 1 MHz).
Figure 8. (a) The equivalent circuit model of the PEDOT film in case of n- and p-doping and and a representation of the different regions of the film. Magnified views of the high frequency domain of PEDOT Nyquist plots at different potentials show the ionic and the electronic resistance of the (b) n-doped and (c) p-doped PEDOT film. Fitting error% values are for R1 (n-doped = 1.7, p-doped =7.5), R2 (n-doped = 2.6, p-doped =12.6), CPE1 (n-doped = 6.8 & 1.2, p-doped =11.4 & 7.9), CPE2 (n-doped = 3.4 & 2.7, p-doped = 1.4 & 0.75) and χ2 (n-doped=0.0047, p-doped=0.0055).
17 Figure 1.
18 Figure 2.
19 Figure 3.
20 Figure 4.
21 Figure 5.
22 Figure 6.
23 Figure 7.
24 Figure 8.
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Supporting Information
Fabrication and Performance of Electrochemically Grafted Thiophene Silicon Nanoparticle Anodes for Li-ion Batteries
Muhammad E. Abdelhamid, Graeme A. Snook*, Joel Gaubicher, Bernard Lestriez, Philippe Moreau, Dominique Guyomard, Anthony P. O’Mullane*
Figure S1. EDAX spectra of Si-thiophene nanoparticles in figure 4d showing the presence S which is an additional evidence on the successful grafting of thiophene to the Si surface.
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Figure S2. Cyclic voltammetry curves of PEDOT showing the p- and n-doping region in; a) solution of 1 M TBAPF6 EC/DMC solution, and b) butyl-methylpyrrolidinium
