Shortly-branched, linear dextrans as efficient binders for silicon/graphite composite electrodes in Li-ion batteries

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silicon/graphite composite electrodes in Li-ion batteries

Zhao, Xiuyun; Yim, Chae-Ho; Du, Naiying; Abu-Lebdeh, Yaser

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Silicon/Graphite Composite Electrodes in Li-Ion Batteries

Xiuyun Zhao, Chae-Ho Yim, Naiying Du, and Yaser Abu-Lebdeh

Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01055 • Publication Date (Web): 11 Jun 2018

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Xiuyun Zhao, Chae Ho Yim, Naiying Du, and Yaser Abu Lebdeh*

National Research Council Canada, Ottawa, ON, K1A 0R6, Canada

* Corresponding author: Yaser.Abu Lebdeh@nrc cnrc.gc.ca

Dextran was evaluated as a binder material for graphite electrode, silicon electrode, and Si/graphite composite electrode in Li ion cells. The performance was also compared with poly(vinylidenefluoride) (PVDF) binder or lithium salt of polyacrylic acid (LiPAA) binder. A graphite rich Si/graphite composite electrode shows a reversible capacity of about 525 mAh g1 at a C/5 rate and has an excellent cycling stability. An interface study using Impedance Spectroscopy, X ray Photoelectron Spectroscopy, and post mortem Scanning Electron Microscopy analysis shows that the superior performance is due to the formation of a solid electrolyte interface (SEI) layer that completely covers the surfaces of silicon and graphite. This could be attributed to a strong interaction between dextran through the shortly branched one dimensional structure and silicon surface as evidenced by Attenuated total reflection Infrared spectroscopy. This along with its low cost and environmental friendly nature makes it an attractive binder material for Si/graphite composite electrodes in Li ion batteries.

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Due to the increasing demand for higher energy and power density lithium ion batteries for hybrid and electric vehicles and large storage for the grid, intensive research has been devoted to developing negative electrode materials with higher capacity than graphite that has a theoretical capacity of 372 mAh g1 (720 Ah L1) with an average voltage of 0.125 V. So far graphite is still the most widely used anode material in commercial lithium ion batteries due to the high initial coulombic efficiency (ICE) and excellent cycling performance. Silicon is considered to be a promising anode material candidate due to its low cost, high theoretical capacity (Li15Si4, 3578 mAh g1, 2194 Ah L1), and appropriate electrode potential (0.4 V).1

However, it is well known that bulk Si electrode loses its capacity after very few charge discharge cycles due to large volume changes leading to cracking and fracture of particles. One of the methods to improve cycling performance is by using nano sized particles that would limit particle fracture and pulverization. It has been observed that Si particles have a threshold size of 150 nm, under which fracture during lithiation could be avoided.2,3 However, it was shown that even for nanosized silicon with appropriate additives such as fluoroethylene carbonate (FEC), the capacity retention and Coulombic efficiency are still not satisfactory due to electrolyte decomposition/film deposition and Li trapping, which result in high contact and solid electrolyte interface (SEI) resistance.4,5 Thus, silicon only electrode is still a challenge to the real world application. The updated version is by designing Si/graphite composite electrodes, in which Si is embedded in a matrix of graphite and all components of the composite are kept in interparticular contact via a suitable binder system.6 Recently, we have calculated the practical energy density improvement of Li ion full cell using Si/graphite composite electrodes and it was found that the maximum improvement is about 17 % and this can be achieved by incorporating 15 25 wt % 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

silicon in graphite.7 This silicon content allows getting a graphite rich composite electrode that could be compatible with the current process conditions.

However, an appropriate binder is very necessary to improve cycling performance of Si/graphite composite electrodes. PVDF and (styrene butadiene rubber) SBR are very popular binders for graphite electrode in industry. A number of binders have been investigated in literature for Si based negative electrodes and stable cycling could be obtained by using polyimide binder or water based binders based on poly(carboxylic acid)s and their alkali metal salts, such as carboxymethyl cellulose (CMC), poly(acrylic acid) (PAA), and alginate.1 The presence of carboxylic acid and hydroxyl functional groups in the polymer backbone and branches allow for hydrogen bonding interactions with silicon surface functional groups compared to the only van der Waals interactions in the conventional PVDF binder. Maximizing the number of functional groups in polymers to form hyper branched network structure to enable multidimensional hydrogen bonding interactions between polymeric binder and silicon has seen as a pathway to improve silicon electrode performance.8

However, it is somewhat complicated to attain good binder for both nano Si and graphite in one composite electrode since graphite and silicon have different surface properties; silicon is hydrophilic while graphite is hydrophobic to say the least. So far, several binder systems have been reported for nano Si/graphite composite electrodes.9 16 Kierzek et al employed conventional PVDF as a binder for the Si/Graphite/Super P/PVDF electrode with a mass ratio of 12/73/5/10. For this electrode, a very low reversible capacity of about 200 mAh g1 was retained after 30 cycles.9 They have also tried NaCMC SBR binder including 3 wt% SBR for the same system and obtained an initial reversible capacity of 620 mAh g1, with average capacity decay of 0.3 % per cycle. They attributed this to a specific composite structure as well as profitable 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

physicochemical properties of the CMC binder.10 Hochgatterer et al. used NaCMC without SBR for Si/graphite composite electrode; the composition ratio was not shown and the capacity was about 1200 mAh g1 in the first cycle and about 800 mAh g1 was retained after 50 cycles.6 Chevrier et al. reported the electrochemical performance of Si (~60 nm)/graphite electrodes with 7.3:59.3 or 20:36 mass ratios using LiPAA binder. In their case, an ICE of up to 86 % and good cycling performance were obtained.11 NaPAA was also used for Si/graphite electrode with 20 wt% Si. A reversible capacity of about 800 mAh g1 was obtained after 30 cycling with 30 wt% NaPAA binder.12 Polyacrylonitrile (PAN)13 and Polyvinyl alcohol (PVA, homemade)14 have also been used for Si/graphite composite electrodes. In both cases, the capacity was changed from about 1200 mAh g1 in the first cycle down to below 600 mAh g1 after 50 cycles.13,14 Yim et al. have tested the 5 wt% Si with ball milled graphite composite electrodes with different binders including NaCMC, NaPAA, PVDF, and polyetherimide (PEI). They found that PEI provided the highest and most stable reversible capacity with more than 500 mAh g1 at C/12 after 350 cycles. This composite also showed good rate capability with a capacity of 200 mAh g1 at 5 C.15 Electrically conductive binders have attracted some attention recently where no carbon additives are required. Zhao et al. have reported conductive poly(1 pyrenemethyl methacrylate) binder used for graphite/nanoSi composite electrode with 20 wt% nanoSi and 70 wt% graphite.16 For this electrode, an areal capacity of above 2.5 mAh cm2 was achieved over 100 cycles.16 Our group recently reported a new conductive polymer as a binder for Si anode or graphite anode and both of them showed successful performance.17 This polymer is based on poly(thiophene) functionalized with ionic carboxylate groups where the poly(thiophene) backbone provides electronic conductivity and the carboxylate ionic groups provide a good interaction with silicon particles or graphite.17 Additionally, with regards to binders for full cell application, LiPAA 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

binder has been reported for graphite/nano Si electrodes to analyze and value electrochemical behavior of full cells.18,19

However, binder research for Si/graphite composite electrode is still in progress and low cost and nontoxic new binders are still desirable. It can be elucidated from the little work available on binders for silicon/graphite composites that PVDF does not work well even in graphite rich composites while polymers with carboxylic/hydroxyl functional groups still offer a successful pathway. Here we report on dextran, a natural polysaccharide industrially extracted from Leuconostoc Mesenteroides bacteria that can offer good battery performance enabled by its predominant one dimensional structure supported by short two dimensional branches. Dextran binder can be used in graphite only electrode, silicon only electrode, and graphite rich Si/graphite composite electrode, unlike hyper branched polysaccharides that work well in silicon only electrodes. It has been found that using dextran as a binder the Si/graphite composite electrode having a 20/65 Si/graphite mass ratio showed excellent cycling performance with a reversible capacity of about 525 mAh g1 up at a C/5 rate to 50 cycles. We believe that dextran can be a promising binder for Si/graphite composite electrodes because it improves lithium ion battery performance in a low cost, sustainable and environmental friendly way.

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&"!" ( " Different electrodes were prepared to characterize the dextran

binder electrochemically. For graphite electrodes, silicon electrodes, and Si/graphite composite electrodes, slurries were made by mixing active materials, Super P carbon black (Timical Carbon, Belgium) and dextran (M.W. 173 K, Sigma) or dextran from

(M.W. 200 K, Fluka) binder in distilled water. TiN (99.7% Alfa Aesar) was used as inert 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

conductive filler in some electrodes in order to measure the electrochemistry of the binder without the presence of an active material. If necessary, iso propanol was added to adjust the wettability to obtain uniform slurries. For comparison, PVDF (HSV 900 Kynar) binder in N methyl 2 pyrrolidone (NMP, Sigma Aldrich, anhydrous 99.5%) and LiPAA binder (lithium salt of polyacrylic acid, as described in Reference 11) in distilled water were also used. Graphite (MAG D, Hitachi) and nano silicon powder (< 100 nm, Aldrich) are considered to be active materials. The ratios and the details are shown in Table 1.

The slurry was mixed using a THINKY Planetary Centrifugal Mixer (ARE 310, Japan) for 10 minutes. A thin layer of slurry was coated onto Cu foil using a 100 or 150 µm stainless steel notch bar. The electrodes were dried in air at 85 °C for 1.5 hours and then transferred to a vacuum oven and dried at 80 °C for 3 days before use. Active material loadings of all electrodes ranged about 2 4 mg/cm2 except for Si electrodes that had about 1 mg/cm2 active material loading.

&"&" ( * " 2325 coin type cells from National

Research Council Canada with two Celgard 2500 separators and a lithium foil common counter/reference electrodes were assembled in an argon filled glove box using the electrodes described above. The electrolyte was a 1 M solution of LiPF6 in ethylene carbonate (EC) /diethyl

carbonate (DEC) / monofluoroethylene carbonate (FEC) [30/60/10 v/v/v]. All cells were cycled with an Arbin BT2000 Test System at 30 °C. A C/10 rate with a 0.1 mA trickle was used for all cells in the first cycle and a C/5 rate with a 0.2 mA trickle in the following cycles in the voltage range of 0.005 0.9 V. The C rate was calculated assuming that Si and graphite are active phases with theoretical capacities of 3578 mAh g1 and 372 mAh g1, respectively. The measured capacities were calculated based on the total mass of Si and graphite in the composite electrodes. 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

For binder/TiN electrodes, binders were assumed active during electrochemical cycling, as shown in Table 1, and a theoretical of 1000 mAh g1 was used based on the experience of Ref 20 and 21. Here 1C is defined as the current to discharge or charge the theoretical capacity in 1 hour.

The electrochemical impedance spectroscopy (EIS) measurements were performed in a frequency range of 150 kHz to 10 mHz with AC amplitude of 10 mV using a SolartronTM /263A analyzer. The EIS data were fitted to the equivalent circuit using CorrWare® electrochemistry software to analyze the individual resistances.

&"+" , ) * " A Hitachi SU5000 Scanning

Electron Microscope (SEM) was used to observe the morphology of the electrode coatings before and after cycling. X ray photoelectron spectroscopy (XPS) spectra were obtained using an AXISUltra instrument from Kratos Analytical and monochromatic Al Kα X rays (1486.6 eV) at 15 kV and 15 mA (emission current); the base pressure within the chamber was approximately 10−8 Pa. The shift in the binding energy was corrected using the C 1s level at 284.8 eV as an internal standard. After 50 discharge/charge cycles, the cells were transferred into an Ar filled glove box and then disassembled using an opener (DPM Solutions Inc.) without short circuit. The coating disks were taken out gently and washed thoroughly with dimethyl carbonate (DMC) to remove the electrolyte and then were sealed in vials in Ar for test. A peel off test was performed for electrode coatings before cycling using 3M Magic Scotch Tape. The coating was cut to 2 cm × 6 cm rectangles. The tape was gently pressed to the coating surface from one edge to the other edge to get a good contact. And then one edge was fixed and the tape was peeled from this edge to the other one.

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Infrared spectra were recorded using Thermo Fisher Nicolet 6700 with a KBr beam splitter from 4000 to 500 cml at 4 cml resolution and 32 scans at room temperature. Attenuated total reflection (ATR) single bounce with diamond crystal was used as an accessory. For test, silicon powder and dextran (80:20 by mass) were mixed in water by a THINKY Planetary Centrifugal Mixer (ARE 310, Japan) for 10 minutes. Some iso propanol was added to get uniform slurry. After that, the mixture was dried at dried in air at 85 °C for 1.5 hours. The resulting product, silicon powder, and dextran were dried in a vacuum oven and dried at 80 °C for 3 days before test.

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Dextran is a natural polysaccharide with short side chains, which contains many functional groups, such as hydroxyl and ether, in each of the mostly linear polymer monomeric units (Figure 1a). It was proved that polysaccharides as binder materials are very promising for Si anodes of lithium ion batteries due to the abundance of hydroxyl or when modified by carboxyl groups.22 It is well known that the native surface of silicon is made up of silicon oxide (Si O Si) and silanol (Si OH) bonds.6,23,24 The carboxylic and hydroxyl groups can be bond to silicon surface with a number of hydrogen bonds or ester like covalent bonds when coating slurry is heated at certain temperature. The resulting structures can favor Si electrode stability. In the case of dextran, as shown in the schematic in Figure 1(b), 2 3 hydroxyls and 1 2 etheric oxygens per monomeric units can provide multi points of contacts and interactions with the oxide or silanol groups at surface of silicon particles and other components in the composite electrode. It was shown that dextran as a hydrophilic polymer can easily attach to silicon and silicon rubber surfaces.25 In addition, the polarity of dextran might favor an interfacial interaction 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

between the polymer binder and the graphite, carbon additive and current collector which can lead to an overall good cycling performance.

Binder/TiN coatings have been prepared and used to evaluate the electrochemistry of binders in Li ion half cells. In such coatings, the binder was coated in a thin layer on conductive TiN particles as it would be in a composite electrode coating.20,21 TiN is inactive to lithium, and the electrochemical behavior of the binder/TiN coating can be used to estimate the performance of binder by a comparison with the same electrode preparation and testing conditions. As it was reported that LiPAA is not an electrochemically active binder and does not contribute to the reversible or irreversible capacity, the electrochemistry testing of LiPAA/TiN coating is not shown here. Figure S1 shows a comparison of voltage curve between a dextran/TiN coating and a PVDF/TiN coating. The two polymers show very similar voltage curve shape. However, when charged to 0.9 V, the dextran shows an irreversible capacity of about 60 mAh g1, which is much lower than that 105 mAh g1 of the PVDF. This irreversible capacity might be caused by solid electrolyte interface (SEI) formation. In the first charge process and the following cycles the dextran/TiN coating delivers a near zero reversible capacity. This is indicative of the outstanding stability of this polymer during electrochemical process.

Figure S2(a b) show the voltage curves of graphite electrodes with dextran and PVDF binders respectively in the first 1.5 cycles. Both electrodes show sharp staging plateaus that are characteristic of graphitic carbon. Compared to the graphite/carbon black/PVDF electrode, the graphite/carbon black/dextran electrode has a lower voltage in the first lithiation process and a little bigger hysteresis in the initial cycling. However, the lithiation voltage becomes similar to that of graphite/carbon black/PVDF electrode upon cycling. In the first cycle, the graphite/carbon black/dextran electrode shows a discharge capacity of 320 mAh g1 and a charge capacity of 288 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

mAh g1 that are lower compared to graphite/carbon black/PVDF electrode, which delivers a discharge capacity of 397 mAh g1 and a charge capacity of 355 mAh g1. However, in the voltage range of 0.005 0.9 V, the in ICE of graphite/carbon black/dextran electrode is up to 90.1 % that is comparable to 89.5 % of graphite/carbon black/PVDF electrode. The cycling performances of the graphite/carbon black/binder electrodes are shown in Figure S2(c d) and both have good cycling performance. After 50 cycles, graphite/carbon black/PVDF electrode and graphite/carbon black/dextran electrode have 99.8 % and 99.1 % efficiency respectively. This demonstrates that dextran might work as well as the conventional PVDF binder with graphite electrode and can be considered as a sustainable and friendly alternative binder with water as a green solvent. Even though PVDF is widely used for graphite electrodes in industry, there is a trend toward replacing it because its solvent NMP can cause risks to the environment and human health. The interactions between the hydroxyl groups of dextran and the edge surface groups of graphite might be responsible for this good performance. The one dimensional structure and short branches of dextran can help support this interaction, unlike hyper branched polymers where such interactions are not favored due to geometrical hindrance.

Recently, LiPAA has been widely used for Si based electrodes and showed a good electrochemical performance. For comparison, Si/carbon black/LiPAA electrode has also been made and tested under the same conditions. Figure S3(a b) show the voltage profiles of nano silicon in electrodes using LiPAA and dextran binders. Compared to the silicon electrode with LiPAA binder, the voltage curve of the silicon electrode with dextran binder is very similar. They show almost the same plateaus and slope profiles during cycling. Both electrodes show a large irreversible capacity that might be caused by large surface area of nano silicon particles since the binders do not likely have too much contribution based on the above results. In 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

addition, both electrodes can deliver a reversible capacity of over 1000 mAh g1, which is much lower than a theoretical capacity of Si (Li3.75Si, 3578 mAh g1). However, the ICE of Si/carbon

black/dextran electrode is about 2 % higher than that of the electrode with LiPAA binder. Figure S3(c d) show the cycling performance of the Si/carbon black/binder with LiPAA and dextran binders, respectively. With similar silicon loading of about 1 mg/cm2, a capacity of over 500 mAh g1 can be retained after 100 cycles at C/5 cycling rate for both electrodes. It was also noted that the capacity increases progressively with cycling before the 25th cycles and then decreases significantly. Currently, the reason for this capacity change during the cycling is not very clear. This could be related to some SEI contribution. A similar behavior has been observed in the conversion type ZnMn2O4 electrode in which the capacity growth is from SEI contribution that

was confirmed by XPS analysis.26 However, more in depth studies are required to unravel the cause of this phenomenon. The good interactions between the hydroxyl groups of dextran and the surface groups of silicon might be responsible for the good performance. The self healing mechanism (loss of interactions through hydrogen bonding during volume expansion and their recovery during shrinkage) could be similar to what was reported for structurally similar polymers.27

The electrochemical performance of dextran was also evaluated as a binder in a Si/graphite composite electrode with 20 wt % Si and 65 wt% MAG D graphite. As a comparison, PVDF and LiPAA were also used in the electrodes with same compositions. Figure 2(a d) show the voltage curves of Si/graphite/carbon black/binder electrodes made with PVDF, LiPAA, and dextran with two different molecular weight binders, respectively. Compared to the other three electrodes, the electrode with PVDF binder has the largest irreversible capacity; the initial discharge/charge capacities are 843/514 mAh g1. The electrode using LiPAA binder 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

shows the highest reversible capacity as well as the highest ICE of 79 % of all four electrodes. For the electrodes with dextran binders, different molecular weight does not show significant effects on the ICE; both electrodes show a reversible capacity that is over 100 mAh g1 higher than that of the electrode with PVDF binder, as well as a 15 % higher ICE. However, it is a little less than that of the electrode with LiPAA binder.

Figure 3(a d) show the dQ/dV curves of the composite electrodes in the 1st, 2nd, and 50th cycles. All composite electrodes show mixed characteristic behavior of both silicon and graphite in the first cycle. For the electrode with PVDF binder, however, the 0.44 V Li15Si4 peak during

charge process disappears from the second cycle, and the graphite peaks become not typical in the 50th cycle. The suppression of Li15Si4 peak and change of graphite peaks indicate the

inactivity of silicon and crystalline structure decay of graphite in the subsequent cycles. By comparison, all other three composite electrodes keep silicon and graphite electrode characteristic during cycling with a little polarization increase.

The cycling performances of these composite electrodes are shown in Figure 4(a d). The electrode with PVDF binder has a 514 mAh g1 reversible capacity in the first cycle, as shown in the voltage curve, and shows very quick fade in the following cycles. It was well known that PVDF is not a good binder for Si electrodes however it is very popular for graphite electrodes in Li cells. Surprisingly, however, for the composite electrode having 65 wt% graphite, the reversible capacity is only about 50 mAh g1 after 50 cycles. Even assuming graphite is the only active material in this composite electrode, the calculated capacity is still much lower than 372 mAh g1. This is consistent with the result of differential capacity of electrode. As mentioned above, Si becomes inactive and graphite structure might be destroyed during cycling. However, the failure mechanism of a graphite rich composite electrode with PVDF binder is not clear and 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

further work is needed to understand it. For the electrode using LiPAA binder, at a C/5 rate, the capacity shows obvious fade in the first 20 cycles and then becomes stable. A reversible capacity of about 480 mAh g1 is retained up to 50 cycles. Both electrodes with dextran binders have much better cycling performance than that of the electrode with PVDF binder. Especially the electrode with Sigma dextran binder shows a reversible capacity of about 525 mAh g1 at a C/5 rate and no obvious fade has been observed from the 2nd cycle to 50th cycle. The electrode with Fluka dextran shows very similar properties to the electrode using LiPAA binder.

To better understand the electrochemical behaviors of these composite electrodes, EIS measurements were performed on the electrodes shown in Figure 2 4. The measurements were taken before cycling and after 50 cycles, as well as at the state of discharge (SOD, discharged to 0.005 V) and different state of charge (SOC, charged to 0.5 V or 0.9 V) in the first cycle and 51st cycle, respectively. Each IS spectrum was collected after waiting a certain amount of time for relaxation after cycling. The Nyquist plots of the electrodes with different binders are shown in Figure 5 7 and the fitted data are shown in Table S1. The inset in Figure 5 displays the proposed equivalent circuit used for the fitting process. This equivalent circuit consists of two resistors and one constant phase element (CPE) and the fitting was performed on each semicircle independently. The fitted impedances using this equivalent circuit agree well with the actual impedance data.

As shown in Figure 5, all the Nyquist plots of fresh electrodes are composed of one semicircle in the high and middle frequency region, and a sloped line in the low frequency region. This semicircle is related to Cdl (double layer capacitor) because no SEI was formed. The

fitted results reveal that before cycling the Rb (bulk resistance) is almost the same between the

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four electrodes: ~3 U. The small Rb indicates the high conductivity of the electrodes and good

connection between coating and the current collectors.

Figure 6 shows the Nyquist plots of electrodes with different binders at full lithiation and different delithiation states in the first cycle. All electrodes when discharged to 0.005 V contain two semicircles and a sloped line. The semicircle in the higher frequency region is related to the formation of SEI and the second one is related to charge transfer. The diameter of each semicircle for all electrodes is similar, indicative of no big difference of RSEI (Solid Electrolyte

Interface resistance) and Rct (charge transfer resistance) of electrodes. The Rct of the electrode

with LiPAA binder has a lower value compared to other electrodes, as shown in Table S1, indicating the facile charge transfer process at the interface between electrode and the electrolyte. When charged to 0.5 V or 0.9 V, all electrodes show two depressed semicircles and a sloped line. Due to the over depression of the second semicircle, it was very difficult to fit the resistance of this part. As a result, only the first one was fitted. It is very clear that the diameters of these semicircles are smaller than that at SOD, indicating a higher conductivity at this stage.

After 50 cycles, as shown in Figure 7, the resistance of PVDF electrode did not go to a large value although the electrode showed very low capacity. This indicates the failure of PVDF electrode should be caused by some reasons other than low conductivity. In the following cycle, a big difference in EIS behavior can be observed between PVDF electrode and other electrodes. PVDF electrode shows one semicircle whatever its state. By contrast, all other three electrodes still contain two depressed semicircles, especially at SOC, just like what they are in the first cycle. This is also consistent with the cycling performance of electrodes.

In order to shed light on the role of dextran binder, the morphologies of the Si/graphite/carbon black/binder coatings before and after cycling were characterized by SEM 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

analysis respectively. Figure 8 and Figure 9 show the SEM images of all composite electrode coatings before and after 50 cycles respectively. It can be observed that these Si/graphite electrodes show a similar rough surface morphology before and after cycling and no obvious structural failure after cycling at a low magnification. At a higher magnification, after cycling, the particle size of Si and carbon black is likely enlarged and some agglomeration can be seen in all electrodes, which could be one reason for the capacity fade. However, these particles are still bonded to the surface of those big graphite chunks that is similar to what is observed before cycling. A simple peel off test was also performed on uncycled Si/graphite composite electrode coatings. Figure S4 shows the photographs of the coatings with different binders before and after peeling off, as well as the tapes after peeling. The PVDF coating indicates the best adhesion to copper foil among all electrode coatings. After peeling, copper substrate was not exposed too much and there is the smallest amount of powder stuck to the tape. This is not surprising since PVDF has been widely used for graphite electrode and has a good adhesion to copper foil. By comparison, LiPAA coating is likely not stuck to the copper foil very firmly. For the coatings with dextran binders, lots of powders were left on the tapes that indicates poor adhesion between the electrode coating and the copper foil. This could be due to the fact that the ether groups which are acetals play no significant role in polymer to metal adhesion. As a result, it is suspected that good cycling performance of dextran binder is caused by good adhesion between binder and active particles instead of copper current collector.

The FTIR spectroscopy was used to characterize the interaction between the dextran binder and Si powder. Figure 10 shows the ATR FTIR spectra of silicon powder, dextran, and silicon/dextran composite (80:20 by mass) respectively. The Si O Si asymmetric stretching band at 1176 cm−1 and Si O Si symmetric stretching band at 885 cm−1 were observed for silicon 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

powder. This indicates the existence of silicon oxide layer on nano silicon. In addition, the peak at 3740 cm1 corresponds to OH groups on the surface of silicon. The dextran shows a strong peak at 1000 cm1 for C O stretching and a broad peak at ∼3420 cm−1 corresponding to O H groups stretching vibrations.28 For the heated Si/dextran composite, which was heated to simulate the heating step in the coating process, the intensity of the hydrogen bonding peaks at

∼3420 cm−1

and 3740 cm1 were significantly weakened. At the same time, a peak at 1100 cm1 that could be ascribed to C O and Si O stretching in the Si/dextran composite is greatly strengthened. Both indicate a strong interaction between dextran and Si surface. Such a strong interfacial interaction is anticipated to be favorable for enhancing the electrode integrity and good electrical network.

XPS was also performed to further investigate this interaction. All Si 2p spectra have been normalized to the background at 107 eV. Figure 11(a) shows Si 2p spectra for the Si/graphite composite electrode coatings with different binders before cycling. All coatings have common features with a strong and broad peak between 100 eV and 105 eV and two weaker peaks between 96 eV and 100 eV. Since the standard XPS values of Si and SiOx are at 99.4 eV

and 103.5 eV respectively, the silicon surface of these composite electrodes probably contains different sub oxides such as Si1+ (100.26 eV), Si2+ (100.9 eV), Si3+ (101.8 eV) and Si4+ (103.5 eV).29 Moreover, the intensity of the strong, board peak is different for different coatings. Dextran coatings have lower intensities than the coatings with other binders. It could be caused by the interaction between C O or OH groups of binders and silicon oxides or OH group on silicon surface. This is also consistent with the ATR FTIR result. For the weak peaks between 96 eV and 100 eV, they might correspond to exposed silicon surface uncovered by the native oxide layer. A slight chemical shift from the standard was observed that could be due to 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

electronegativity effects caused by some functional groups such as Si H or Si C. Figure 11(b) shows Si 2p spectra of electrode after 50 cycles. It was found that when using dextran binders, the intensities of both Si peaks and silicon oxides peaks were suppressed compared to the other two coatings that indicates the formation of thicker SEI of dextran electrodes that completely covers the silicon surface. The other two binders still show strong peaks corresponding to silicon and silicon oxides with much higher intensity. This is consistent with the EIS results as the sample with Sigma dextran binder showed much higher RSEI compared to the other binders.

However, the sample with Fluka dextran binder showed lower resistance. This is due to the contribution from the lithium counter electrode. Further study is needed to separate the contribution of each electrode to the total resistance in a mean of three electrode cell.

-" $# % $#

Dextran was found to be an efficient binder for graphite electrode, silicon electrode, and Si/graphite composite electrode in Li ion half cells. Dextran shows a low initial irreversible capacity and an excellent electrochemical stability. When employed as a binder in graphite electrode or silicon electrode, both electrodes show good cycling performance, which is comparable with conventional binders (LiPAA, PVDF). It was also found that the cycling performance of the Si/graphite composite (20:65 by mass) electrode using dextran binder is better than that of the LiPAA ionic binder. SEM analysis shows that the dextran binder provides good adhesion and mechanical stability between active particles even after cycling despite the poor adhesion to the current collector. FTIR and XPS results indicate the good performance could be caused by the function groups’ interaction between dextran binder and active particles enabled by the shortly branched, one dimensional polymer structure. Future work will focus on 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

the use of a combination binder of dextran/SBR where the SBR latex could provide strong adhesion to the graphite particles as well as the copper current collector to improve the performance of dextran binder for Si/graphite composite electrodes.

.#$/ ) #

The authors acknowledge the funding from the Office of Energy Research and Development at Natural Resources Canada. The authors also thank Oltion Kodra (National Research Council Canada, ON, Canada) for his assistance towards XPS data acquisition and Gilles Robertson (National Research Council Canada, ON, Canada) for his help in taking IR measurement.

0 ,

Summary of the fitted parameters from EIS data; Electrochemistry of dextran electrode, graphite electrode and silicon electrode; Peel off tests of Si/graphite composite electrode coatings.

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

1 #

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: 1 ,0

: !" Electrode formulations and cycling C rate for cells

1 ,0 !" (a) The chemical structure of dextran where the dotted line signify the main chain and

the bottom dotted line on the left signifies the branches; (b) The schematic representation of the interaction between dextran and silicon particles.

1 ,0 &" Voltage curves of Si/graphite composite coatings with PVDF, LiPAA, and dextran

binders respectively.

1 ,0 +" Differential capacity curves of Si/graphite composite coatings with PVDF, LiPAA,

and dextran binders respectively.

1 ,0 -" Capacity performance of Si/graphite composite coatings with PVDF, LiPAA, and

dextran binders respectively.

1 ,0 3" Niquist plots of Si/graphite/carbon black/binder electrodes before cycling.

1 ,0 6" Niquist plots of Si/graphite/carbon black/binder electrodes at different states in the

first cycle.

1 ,0 4" Niquist plots of Si/graphite/carbon black/binder electrodes at different states in the

51st cycle. 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

1 ,0 5" SEM images of Si/graphite/carbon black/binder electrode coatings with PVDF (a b),

LiPAA (c d), Dextran (Sigma) (e f), and dextran (Fluka) (g h) binders before cycling.

1 ,0 9" SEM images of Si/graphite/carbon black/binder electrode coatings with PVDF (a b),

LiPAA (c d), Dextran (Sigma) (e f), and dextran (Fluka) (g h) binders, after cycling.

1 ,0 !2" FTIR spectra of Si powder, dextran (Sigma), and Si/ dextran (Sigma) composite after

heating.

1 ,0 !!" Si 2p spectra of Si/graphite/carbon black/binder electrode coatings with different

binders (a) before and (b) after cycling. 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

: 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

Table 1. Electrode formulations and cycling C rate for cells

Coating Compositions Weight Ratio Active Material 1C (mAh g-1₎

graphite/carbon black/PVDF 90/2/8 graphite 372

graphite/carbon black/dextran 90/2/8 graphite 372

Si/carbon black/LiPAA 85/5/10 Si 3578

Si/carbon black/dextran 85/5/10 Si 3578

Si/graphite/carbon black/PVDF 20/65/5/10 Si and graphite 1126

Si/graphite/carbon black/LiPAA 20/65/5/10 Si and graphite 1126

Si/graphite/carbon black/dextran (Fluka) 20/65/5/10 Si and graphite 1126

Si/graphite/carbon black/dextran (Sigma) 20/65/5/10 Si and graphite 1126

PVDF/TiN 5/95 PVDF 1000

dextran (Sigma)/TiN 5/95 dextran 1000

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

Figure 1. (a) (b) Main chain Branch 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

Figure 2. (a) (b) (c) (d) 0 200 400 600 800 1000 Capacity / mAh g-1 0 0.2 0.4 0.6 0.8 1 Po te n ti a l / V Si/MAG-D, PVDF 1st Charge: 514 mAh g-1 1st Discharge: 843 mAh g-1 ICE = 60.9 % 0 200 400 600 800 1000 Capacity / mAh g-1 0 0.2 0.4 0.6 0.8 1 Po te n ti a l / V Si/MAG-D, LiPAA 1st Charge: 715 mAh g-1 1st Discharge: 904 mAh g-1 ICE = 79.1 % 0 200 400 600 800 1000 Capacity / mAh g-1 0 0.2 0.4 0.6 0.8 1 P o te n ti a l / V

Si/MAG-D, Dextran (Sigma)

1st Charge: 636 mAh g-1 1st Discharge: 825 mAh g-1 ICE = 77.1 % 0 200 400 600 800 1000 Capacity / mAh g-1 0 0.2 0.4 0.6 0.8 1 P o te n ti a l / V

Si/MAG-D, Dextran (Fluka)

1st Charge: 665 mAh g-1 1st Discharge: 875 mAh g-1 ICE = 76.1 % 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

Figure 3. (a) (b) (c) (d) 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

Figure 4. (a) (b) (c) (d) 0 10 20 30 40 50 Cycle Number 0 200 400 600 800 1000 Ca p a c it y / m A h g -1 Si/MAG-D, PVDF

Active material loading 3.67 mg cm-2 0 10 20 30 40 50 Cycle Number 0 200 400 600 800 1000 Ca p a c it y / m A h g -1 Si/MAG-D, LiPAA

Active material loading 3.77 mg cm-2 0 10 20 30 40 50 Cycle Number 0 200 400 600 800 1000 C a p a c it y / m A h g -1

Active material loading 2.38 mg cm-2

Si/MAG-D, Dextran (Sigma)

0 10 20 30 40 50 Cycle Number 0 200 400 600 800 1000 Po te n ti a l / m A h g -1

Active material loading 3.07 mg cm-2

Si/MAG-D, Dextran (Fluka) 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

Figure 5. (a) (b) (c) (d) 0 200 400 600 Z' /  0 200 400 600 -Z " /  Before cycling Si/MAG-D, PVDF 5 Hz 150 KHz 0.5 Hz 0 100 200 300 Z' /  0 100 200 300 -Z " /  Before cycling Si/MAG-D, LiPAA 150 KHz 24 KHz 2 KHz 0 200 400 600 800 1000 Z' /  0 200 400 600 800 1000 -Z " /  Before cycling

Si/MAG-D, Dextran (Sigma)

150 KHz 10 Hz 0.75 Hz 0 200 400 600 800 Z' /  0 200 400 600 800 -Z " /  Before cycling

Si/MAG-D, Dextran (Fluka)

150 KHz 7.5 Hz 3 Hz 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

Figure 6. (a) (b) (c) (d) 0 20 40 60 80 Z' /  0 20 40 60 80 -Z " /  Discharged to 0.005 V Charged to 0.5 V Charged to 0.9 V Si/MAG-D, PVDF 150 KHz 95 Hz 0.5 Hz 0.1 Hz 0.1 Hz 0.1 Hz 189 Hz 3 Hz 0 20 40 60 Z' /  0 20 40 60 -Z " /  Discharged to 0.005 V Charged to 0.5 V Charged to 0.9 V Si/MAG-D, LiPAA 150 KHz 0.015 Hz 0.01 Hz 0.01 Hz 0.5 Hz 95 Hz 48 Hz 1.5 Hz 3 Hz 0 30 60 90 120 150 180 Z' /  0 30 60 90 120 150 180 -Z " /  Discharged to 0.005 V Charged to 0.5 V Charged to 0.9 V

Si/MAG-D, Dextran (Sigma)

150 KHz 0.01 Hz 0.01 Hz 0.01 Hz 0.3 Hz 75 Hz 1 Hz 1 Hz 95 Hz 0 15 30 45 60 75 Z' /  0 15 30 45 60 75 -Z " /  Discharged to 0.005 V Charged to 0.5 V Charged to 0.9 V

Si/MAG-D, Dextran (Fluka)

150 KHz 0.01 Hz 0.01 Hz 0.01 Hz 5 Hz 75 Hz 60 Hz 3 Hz 3 Hz 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

Figure 7. (a) (b) (c) (d) 0 50 100 150 200 Z' /  0 50 100 150 200 -Z " /  After 50 cycles Discharged to 0.005 V Charged to 0.5 V Charged to 0.9 V Si/MAG-D, PVDF 150 KHz 15 Hz 30 Hz 0.02 Hz 0.01 Hz 0.01 Hz 0.01 Hz 0 20 40 60 80 Z' /  0 20 40 60 80 -Z " /  After 50 cycles Discharged to 0.005 V Charged to 0.5 V Charged to 0.9 V Si/MAG-D, LiPAA 150 KHz 0.01 Hz 0.01 Hz 0.01 Hz 0.01 Hz 5 Hz 6 Hz 20 Hz 7.5 Hz 0 40 80 120 Z' /  0 40 80 120 -Z " /  After 50 cycles Discharged to 0.005 V Charged to 0.5 V Charged to 0.9 V

Si/MAG-D, Dextran (Sigma)

150 KHz 0.01 Hz 0.01 Hz 0.01 Hz 0.01 Hz 0.1 Hz 10 Hz 0.3 Hz 30 Hz 10 Hz 30 Hz 0 30 60 90 120 Z' /  0 30 60 90 120 -Z " /  After 50 cycles Discharged to 0.005 V Charged to 0.5 V Charged to 0.9 V

Si/MAG-D, Dextran (Fluka)

150 KHz 0.01 Hz _{0.01 Hz} 0.01 Hz 0.01 Hz 10 Hz 10 Hz 15 Hz 10 Hz 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

Figure 8. 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

Figure 9. 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

Figure 10.

4000 3500 3000 2500 2000 1500 1000

500

Wavenumber / cm

T

ra

ns

mit

ta

nc

e

(a) silicon

(c) silicon-dextran

(b) dextran

SiO-H

stretching

_Si-O-Si

stretching

Si-O-Si

bending

C-O

stretching

O-H

stretching

C-H

stretching

Figure 11. (a) (b) 108 104 100 96 92 Binding Energy / eV Int en si ty / a. u. PVDF LiPAA Dextran (Sigma) Dextran (Fluka) Si 2p, before cycling Si SiO_x 108 104 100 96 92 Binding Energy / eV Int en si ty / a. u. PVDF LiPAA Dextran (Sigma) Dextran (Fluka) Si 2p, after cycling Si SiO_x 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55