Comparative Study of Thermally Treated Silicon Anode Nanostructures Based on Polyacrylic Acid and Polyaniline for Lithium Ion Battery Application

Proceedings of the South Dakota Academy of Science, Vol. 94 (2015) 253

COMPARATIVE STUDY OF THERMALLY TREATED SILICON ANODE NANOSTRUCTURES BASED ON POLYACRYLIC ACID AND POLYANILINE FOR LITHIUM ION BATTERY APPLICATION

Michael McGraw1, Praveen Kolla1, Rob Cook2, James Wu3, Vadim Lvovich3, and Alevtina Smirnova1* 1Department of Chemistry and Applied Biological Sciences South Dakota School of Mines and Technology Rapid City, SD 57701 2Zyvex Technologies Rapid City, SD 57701 3NASA Glenn Research Center Cleveland, OH 44135 *Corresponding author email: [email protected]

ABSTRACT

A comparative study between a nonconductive thermoplastic, polyacrylic acid (PAA), and an electronic-conducting polymer, polyaniline (PANI), has been conducted related to their function as binders for Silicon Nanoparticle (SiNP)-based anodes in Li-ion battery application. SEM analysis was used to understand the morphological differences associated with dispersion of PAA/PANI with SiNPs. Further, thermogravimetric analysis was conducted in order to understand the thermal degradation and to choose their associated heat-treatment temperatures of 400 oC (PAA) and 300 oC (PANI). The electrochemical performance of the resultant silicon-based electrodes assembled in Lithium-half-cell configuration was evaluated in terms of their specific capacitance and cycle-stability. The results indicated that PANI-based SiNP anodes heat-treated at 300 oC demonstrate the highest specific capacitance and the lowest capacity loss over 10 cycles, which could be attributed to higher electrical conductivity and improved adhesion.

INTRODUCTION

Silicon has attracted substantial attention in lithium ion battery (LIB) research due to its exceptional theoretical specific capacitance (4200 mAhg-1) which is the highest among any known material. However, Si undergoes a 300-400% volume expansion during the lithiation process which causes mechanical deterioration of the silicon macrostructure and loss of LIB performance. Furthermore, loss of electrical conductivity of silicon particles during delithiation implies that the conductivity factor should be considered at nanoscale. Finally, the solid electrolyte interface (SEI) layer (Kawamura, Okada, & Yamaki, 2006) formed on the anode surface and within the anode interior introduces additional challenges, 254 Proceedings of the South Dakota Academy of Science, Vol. 94 (2015) such as reduced Li-ion transport, separation of silicon nanoparticles from the conductive carbon and/or polymer phase, and anode delamination from the Cu support. These challenges can be addressed by (1) using silicon nanostructures (<50 nm) that sustain their mechanical integrity during lithiation-delithiation and (2) choosing an appropriate binder with sufficient electric conductivity, high elastic modulus, and mechanical strength to keep Si-nanoparticles in intimate contact with the conducting phase. Conventional binders, such as polyvinyli- dene fluoride (PVDF), carboxymethyl cellulose (CMC), sodium salts of CMC, combinations of Na-CMC with butadiene block copolymers, and polyacrylic acid (PAA) (Magasinski et al. 2010) partially eliminate the problems and improves LIB performance and cycling behavior. Conductive polymers such as polyaniline (Li et al. 2014; Wang et al. 2013) and poly(3,4-ethylenedioxythiophene) (Kim et al. 2014) were applied for both LIB cathode and anode materials to improve conductivity and provide mixed ion-electronic pathways for Li cations and electron transport. SiNP-PANi half cells (Wu et al. 2013) with 1600 mAhg-1 capacity at high currents (up to 6 Ag-1) demonstrated close to zero capacity loss after 1000 cycles. However, the SiNP-PANI half-cell specific capacitance (1600 -1g ) was significantly lower than Si-nanotubes (Cuiet al. 2009) (3200 mAhg-1) or nanowires (4277 mAhg-1). Earlier it was shown that thermal treatment of PANI, PAA, and PANI/PAA blends (Lu eta al. 2003) can significantly change their electrical conductivity (3.2 mS.cm-1 for polyacrylic acid/graphite nano-platelet (Tang et al. 2009) and PANI ~103 -102 mS.cm-1). The purpose of this study was to identify the effects of heat-treatment on the SiNP based anodes prepared with two binders, specifically PAA and PANI, and characterize the effects in terms of electrochemistry. The chemistries involved with heat treatment of PAA and PANI are starkly different. Heat treatment of PAA results in the partial dehydration and in some cases decarboxylation of the carboxylic acid groups. Heat treatment of PANI, specifically the PANI-phytic acid hydrogel previously introduced (Wu et al. 2013), results in the oxidation of phosphate groups found on phytic acid and the evolution of water. In both cases, the eliminated functional groups are acidic in nature, which, in an electrochemical cell, can result in the activation of organic carbonates used as electrolyte solvents. This increases the likelihood of electrolyte reduction and the deposition of insoluble products (SEI) onto the anode surface . It is our hypothesis that elimination of acidic functional groups by heat treatment may have a beneficial effect on cell performance by minimizing electrode-electrolyte interaction.

METHODS

The SiNP-based anode slurries with PAA as a binder were synthesized by homogenization of 0.2 g SiNP (US Research Nanomaterials) with 5 mL of distilled water (2 hr) followed by addition of a 2.5 g of graphitized carbon and exfoliated carbon in 1:1 weight ratio under stirring conditions (2 hr). An aqueous solution of PAA from Alfa Aesar (Av. M.W. 240,000) in distilled water was prepared and added to the initial SiNP-water mixture under stirring conditions resulting in an SiNP-PAA slurry. The SiNP-PANi nanocomposites were processed in three steps. Proceedings of the South Dakota Academy of Science, Vol. 94 (2015) 255

First, 0.08 g of SiNP (US Research Nanomaterials) were homogenized with distilled water. Then, 0.00837 g of the aniline monomer (Sigma-Aldrich) and 0.01782 g aqueous phytic acid solution (50% by wt., Sigma-Aldrich) were added to the SiNP-water suspension and sonicated. Finally, 0.00856 g of ammonium persulfate (Sigma-Aldrich) was added to initiate radical polymerization of the ox- idized aniline. After about 3 minutes of continuous stirring, we observed a color change from brown to green indicating formation of an electrically conductive emeraldine. The slurries were deposited onto carbon coated Cu foil (MTI) using a Systematic Automation screen printer and allowed to dry under ambient conditions for 1 hour. Once dried, the PANI electrodes were soaked in deionized water for 1 hour to remove excess phytic acid. The electrodes were heated in a vacuum oven at various temperatures and constant pressure covered with Teflon and a stainless steel plate. The electrodes were cut into disks (active area 0.907 cm2) and degassed overnight in a vacuum. CR2023 stainless steel coin cells with a K1640 Celgard polyethylene membrane (16 µm thick) were assembled in an argon glove box with O2 and H2O concentrations < 1 ppm. Lithium foil reference electrodes (Alfa Aesar) were used with SiNP-PANi and SiNP-PAA working electrodes. During the half-cell assembly, 30 µL of an electrolyte solution from Solvionic containing 1.0 M LiPF6 in 1:1 v/v ethylene carbonate: dimethyl carbonate with 2% wt. vinylene carbonate was deposited on both sides of the separation membrane. Thermogravimetric analysis was performed using an SDT Q600 (TA In- struments). Impedance spectroscopy data were obtained from PARStat 2273 (Princeton Applied Research). Galvonostatic charge/discharge cycling and cyclic voltammetry measurements were performed using 580 Battery (Scribner Associ- ates). The images of the electrode surfaces and nanocomposites were taken using a Supra 40VP (Zeiss) field emission scanning microscope equipped with EDX.

RESULTS AND DISCUSSION

Morphological characterization—The SEM images for SiNP-PAA and SiNP-PANI (Figure 1) indicate satisfactory coverage of the Cu support and an even distribution of the silicon nanoparticles with a mean diameter of about 50 nm within the anode layer. The “in-situ” PANI impregnation resulted in higher dispersion and better distribution than the simple stirring process used for PAA. The SiNPs-C-PAA nanocomposite revealed “islands” of PAA indicated by the circle in Figure 1. Thermogravimetric analysis (TGA) and dynamic scanning calorimetry (DSC) were performed on SiNP-PAA and SiNP-PANI composites to elucidate the temperatures at which their acidic groups evolve. The results of TGA/DSC are presented in Figure 2. In the case of PANI, the significant weight loss below 100o C corresponds to the evaporation of moisture and ambient monoatomic molecules. The steady weight loss between 100 and 300o C corresponds to the decomposition/evaporation of synthesis byproducts (e.g. ammonium persulfate, ammonia, sulfuric acid, and aniline). The massive drop in weight percent between 300 and 400 oC which parallels the DSC peak at 362 oC most likely corresponds to the hour to remove excess phytic acid. The electrodes were heated in a vacuum oven at various temperatures and constant pressure covered with Teflon and a stainless steel plate. The electrodes were cut into disks (active area 0.907 cm2) and degassed overnight in a vacuum. CR2023 stainless steel coin cells with a K1640 Celgard polyethylene membrane (16 µm thick) were assembled in an argon glove box with O2 and H2O concentrations < 1 ppm. Lithium foil reference electrodes (Alfa Aesar) were used with SiNP-PANi and SiNP-PAA working electrodes. During the half-cell assembly, 30 µL of an electrolyte solution from Solvionic containing 1.0 M LiPF6 in 1:1 v/v ethylene carbonate: dimethyl carbonate with 2% wt. vinylene carbonate was deposited on both sides of the separation membrane. Thermogravimetric analysis was performed using an SDT Q600 (TA Instruments). Impedance spectroscopy data were obtained from PARStat 2273 (Princeton Applied Research). Galvonostatic charge/discharge cycling and cyclic voltammetry measurements were performed using 580 Battery (Scribner Associates). The images of the electrode surfaces and nanocomposites were taken using a Supra 40VP (Zeiss) field emission scanning microscope equipped with EDX.

RESULTS AND DISCUSSION

Morphological characterization--The SEM images for SiNP-PAA and SiNP-PANI (Figure 1) indicate satisfactory coverage of the Cu support and an even distribution of the silicon nanoparticles with a mean diameter of about 50 nm within the anode layer. The “in-situ” PANI impregnation resulted in higher dispersion and better distribution than the simple stirring process used256 for PAA. ThePr oceedingsSiNPs-C-PAA of thenanocomposite South Dakota revealed Academy “islands” of ofScience, PAA indicated Vol. 94 by(2015) the circle in Figure 1.

Figure 1. FESEM images for non-heat treated (A) SiNPs-C-PAA and (B) SiNP-PANI deposited on Figurecopper 1. support. FESEM Islands images of for PAA non- areheat enclosedwithin treated (A) SiNP the ovals-C- PAAin A. and (B) SiNP-PANI deposited on copper support. Islands of PAA are enclosedwithin the oval in A.

Figure 2. TGA and DSC profiles of (A) SiNP-PAA and (B) SiNPs-PANI inks. Figure 2. TGA and DSC profiles of (A) SiNP-PAA and (B) SiNPs-PANI inks. Thermogravimetric analysis (TGA) and dynamic scanning calorimetry (DSC) were performed on SiNP-PAA and SiNP-PANI composites to elucidate the temperatures at which their acidic groups evolve. The results of TGA/DSC are presented in Figure 2. In the case of PANI,thermal the significantdehydration weight of lossphytic below acid, 100 owhichC corresponds coincides to the with evaporation the recent of moisture study and by ambientDaneluti monoatomic and Matos molecules. 2013. The steady slight weight drop loss in betweenweight 100percent and 300 betweenoC corresponds 500 and to the600 decomposition/evaporation oC corresponds to the of partial synthesis pyrolysis byproducts of phytic(e.g. ammonium acid. Based persulfate, on these ammonia, results, sulfuricthe chosen acid, and heat-treatment aniline). The massive temperature drop in weight for SiNP-PANI percent between was 300 300and 400oC. o CUpfieldwhich parallelsshifts in the the DSC DSC peak peaks at 362 wereoC most assumed likely corresponds to be present to the due thermal to the dehydration 10 oC per of phyticminute acid,scan which rate. coincidesBecause withof this, the recent 300 ostudyC was by chosen Daneluti as and thereMatos should 2013. beThe sufficient slight drop in energy weight percent between 500 and 600 oC corresponds to the partial pyrolysis of phytic acid. Basedto initiate on these the results, dehydration the chosen of heat phytic-treatment acid, temperature yet avoid forthe SiNP side-PANI reaction was 300seenoC. at 475 o UpfieldC where shifts the in theweight DSC peakspercent were slightly assumed increases,to be present suggesting due to the 10 oxidation oC per minute of silicon.scan rate.In theBecause case ofof this, SiNP-PAA 300 oC was anodes, chosen as the there heat should treatment be sufficient temperature energy to initiate was thechosen dehydrationto be 400 ofoC phytic to parallel acid, yet the avoid strong the side DSC reaction peak seen which at 475 mostoC where likely the weightrepresents percent the slightlydehydration/decarboxylation increases, suggesting oxidation of the of silicon. PAA. HeatIn the treatedcase of SiNP anodes-PAA wereanodes compared, the heat to treatment temperature was chosen to be 400 oC to parallel the strong DSC peak which most likelya control represent groups the of dehydration/ non-heatdecarboxylat treated SiNP-PAAion of the PAA. and SiNP-PANI Heat treated anodes anodes. were We hy- comparedpothesized to a that control degradation group of non- ofheat these treated acidic SiNP groups-PAA andmay SiNP-PANI improve cellanodes. performanceWe hypothesizedby decreasing that the degradation amount of of these electrolyte acidic groups decomposition may improve celland performance SEI formation. by decreasingElectrochemical the amount ofcharacterization— electrolyte decompositionHeat and treated SEI formation. and non-heat treated SiNP- Electrochemical characterization--Heat treated and non-heat treated SiNP-PAA and PAA and SiNP-PANI half cells were galvanostatically charged and-1 discharged at SiNP0.1 Ag-PANI-1 from half 0.1cells to were 1 V. galvanostatically The specific charged capacitance and discharged values asat 0.1 well Ag as fromcoulombic 0.1 to 1 V-effi . The specific capacitance values as well as coulombic efficiency can be seen for the first 10 cyclesciency in can Figure be 3.seen For for all cells,the first heat treated10 cycles and innon-heat Figure treated, 3. For the all initial cells, specific heat treated capacitance and -1 wasnon-heat approximately treated, 2000 the mAhg initial-1 withspecific the exception capacitance of non- heatwas approximatelytreated SiNP-PANI 2000 which mAhg was approximately 2300 mAhg-1. However, initial specific capacitance values can be misleading since a fraction of that value is contributed by the reduction of the organic electrolyte solvent by the Li reference electrode to form SEI. From an engineering perspective, minimizing capacitance loss after the first cycle is more important for anode performance than striving to achieve exceedingly high specific capacitance values. The demand for batteries that exhibit greater stability and cycle life greatly outweighs the demand for higher specific capacitance, especially since cathodes, not anodes, are the capacitance defining electrodes in a full cell. In all of our cells, the original specific capacitance values drop after the first cycle. This difference can Proceedings of the South Dakota Academy of Science, Vol. 94 (2015) 257 with the exception of non-heat treated SiNP-PANI which was approximately 2300 mAhg-1. However, initial specific capacitance values can be misleading since a fraction of that value is contributed by the reduction of the organic electrolyte solvent by the Li reference electrode to form SEI. From an engineering perspective, minimizing capacitance loss after the first cycle is more important for anode performance than striving to achieve exceedingly high specific capacitance values. The demand for batteries that exhibit greater stability and cycle life greatly outweighs the demand for higher specific capacitance, especially since cathodes, not anodes, are the capacitance defining electrodes in a full cell. In all of our cells, the original specific capacitance values drop after the first cycle. This difference can be accounted for by electron exchange between the reference electrode and the electrolyte solvent. For all non-heat treated cells, the capacity continues to drop even after the first cycle and every cycle thereafter. We hypothesize that this continual drop in capacitance is due to the catalyzation of further electrolyte reduction by the acidic groups in the binder. Greater amounts of insoluble SEI products deposited on the surface of the working electrode accelerates the rate at which the open circuit potential of the cell drops during the lithiation process. Therefore, the potential for Li insertion into the bulk of the SiNP diminishes as well. In the case of SiNP-PAA, carboxylic acid groups in polyacrylic acid donate a proton to activate the organic carbonates found in the electrolyte solvent. In the case of SiNP-PANI, it is the phosphate groups found in phytic acid that donate protons. It was expected that heat treatment would weaken this catalytic effect. However, it is apparent that for heat treated SiNP-PAA cells, the specific capacitance violently drops after every cycle and approaches zero. This poor performance is due most likely to the loss of ion-conductivity throughout the electrode. The ion conductivity of PAA is directly dependent on cation exchange through the carboxylic acid group. When Li ions cannot conduct through the interior of the electrode, the cell voltage quickly drops before maximum capacity is reached. Therefore, heat treatment of PAA does not improve the performance of the SiNP-PAA electrode. For heat treated SiNP-PANI cells, the ion conductivity of polyaniline is not dependent on the acidic groups found in phytic acid. Therefore, Li cations can still make their way into the interior of the Si bulk. The flat line specific capacitance following the first cycle shows that a limited amount of Li is irreversibly consumed and the open circuit potential does not change from cycle to cycle. Another hypothesis is that the composites undergo morphological changes when subjected to heat treatments. PAA has a glass transition temperature of 106 oC and PANI has a glass transition of 160-300 oC (Qi et al. 2009) depending on the doping level, anion, chain length, and crosslinking. Heat treatment of the polymer binders may cause nanostructure changes, such as the amount of surface contract between the SiNP and the polymer. This may impact the ion and electron transport processes in these electrodes. In the case of SiNP-PAA, this effect would be harmful, because PAA covering the SiNP surface would block ion and electron transport. In the case of SiNP-PANI, this effect may be beneficial as it would improve electron transport to the SiNP. The improved performance of SiNP-PANI can also be explained by the in-situ synthesis process which promotes a high level of homogenization and thorough 258 Proceedings of the South Dakota Academy of Science, Vol. 94 (2015)

Figure 3. Galvonostatic charge-discharge cycling at 0.1 Ag-1 of (A) Non-treated SiNP-PAA, (B) Figure 3. Galvonostatic charge-discharge cycling at 0.1 Ag-1 of (A) Non-treated SiNP-PAA, (B) Non-treated SiNP-PANI, (C) SiNP-PAA heat-treated at 400 oC and (D) SiNP-PANI heat-treated Non-treated SiNP-PANI, (C) SiNP-PAA heat-treated at 400 oC and (D) SiNP-PANI heat-treated at 300 oC in terms of their charge-discharge, coulombic efficiency and cycle stability. at 300 oC in terms of their charge-discharge, coulombic efficiency and cycle stability.

The improved performance of SiNP-PANI can also be explained by the in-situ synthesis processdispersion which ofpromotes polyaniline a high levelthroughout of homogenization the electrode and thorough interior. dispersion The ofsame polyaniline level of throughouthomogenization the electrode cannot interior. be achievedThe same levelby simply of homogenization mixing silicon cannot nanoparticles be achieved by with simplya polymer, mixing siliconsuch as nanoparticles is the case with with a polymer polyacrylic, such as acid. is the Thiscase with distinction polyacrylic is acid clearly. This distinction is clearly visible in SEM images, where islands of polyacrylic acid are found remotelyvisible fromin SEM the SiNP images,. Furthermore, where phyticislands acid of has polyacrylic the ability to acid oxidize are multiple found polyaniline remotely chains,from thereforethe SiNP. crosslinking Furthermore,(Figure 4)phyticthem and acid creat hasing athethree ability-dimensional to oxidize “caged” multiple nanostructurepolyaniline that chains, confines therefore each silicon crosslinking nanoparticle (Figure to a limited 4) themvolume and (Pan creating et al. 2012; a three-Wu et al.dimensional 2013). The caged “caged” nanostructure nanostructure helps control that volumeconfines expansion each bysilicon inhibiting nanoparticle outward to a growthlimited and volume, in turn, (Panminimizing et al. discontinuity2012; Wu etcaused al. 2013).by fracture Theand caged pulverization. nanostructure The helps homogenouscontrol volume nature ofexpansion the polymer by network inhibiting ensures outward that there growth is Si-polymer and, in interface turn, throughoutminimiz- the electrode interior, promoting stable electron/ion pathways to each Si nanoparticle in the structure.ing discontinuity caused by fracture and pulverization. The homogenous nature of the polymer network ensures that there is Si-polymer interface throughout the electrode interior, promoting stable electron/ion pathways to each Si nanoparticle in the structure. Additional evidence for the advantages of PANI- vs. PAA-based systems was obtained from the cyclic voltammetry (CV) data (Figure 5A and Figure 5B, respectively) that provided valuable information about the morphological changes within silicon-lithium alloy upon lithiation and delithiation, ionic and electronic transport at the interface of SiNP and solid electrolyte interface (SEI), and an indication of the ionic and electronic transport at the interface of SiNP and the binding polymer. Proceedings of the South Dakota Academy of Science, Vol. 94 (2015) 259

Figure 4. A fragment of the cross-linked PANI-phytic acid network within SiNP-PANI nanocomposite.

Additional evidence for the advantages of PANI- vs. PAA-based systems was obtained fromFigure the 4. cyclicA fragment voltammetry of the (CV) cross-linked data (Figure PANI-phytic 5A and Figure acid network5B, respectively) within SiNP-PANI that provided nano- Figure 4.valuablecomposite.A fragment information of the about cross-linked the morphological PANI-phytic changes acid within network silicon- withinlithium alloySiNP-PANIupon nanocomposite.lithiation and delithiation, ionic and electronic transport at the interface of SiNP and solid electrolyte interface (SEI), and an indication of the ionic and electronic transport at the interface Additionalof SiNP and evidence the binding for polymer. the advantages of PANI- vs. PAA-based systems was obtained from the cyclic voltammetry (CV) data (Figure 5A and Figure 5B, respectively) that provided valuable information about the morphological changes within silicon-lithium alloy upon lithiation and delithiation, ionic and electronic transport at the interface of SiNP and solid electrolyte interface (SEI), and an indication of the ionic and electronic transport at the interface of SiNP and the binding polymer.

Figure 5. Cyclic voltammetry plots for PAA-based working electrode (50%SiNP-35%C- Figure 5. Cyclic voltammetry plots for PAA-based working electrode (50%SiNP-35%C-15%PAA) 15%PAA) (A) and PANI-based working electrode (50%SiNP-50%PANI) (B) in CR2023 (A) and PANI-based working electrode (50%SiNP-50%PANI) (B) in CR2023 stainless steel coin stainless steel coin cells vs. Li/Li+ reference electrode at room temperature. cells vs. Li/Li+ reference electrode at room temperature. In the case of PAA (Figure 5A), a small anodic peak at 0.25 V corresponds to delithiation of the carbon phase and the flow of electrons into the SiNPs-C-PAA working electrode. It is necessaryIn the tocase emphasize, of PAAthat (Figure this peak 5A), is visible a small from anodic the very peak beginning at 0.25 (Cycle V corresponds 1), representing to highdelithiation reactivity offor theLi-ion carbon insertion phase into andthe carbon the flow phase ofas opposedelectrons to intothe silicon the SiNPs-C-PAAphase, which Figure 5.doesworkingCyclic not presentvoltammetry electrode. a peak untilIt plots is Cycle necessary for 5PAA-basedat 0.52 toV. emphasize,However, working with electrodethat the this growing (peak50%SiNP number is visible-35%C- of cycles from, 15%PAA)the( A)second and anodic PANI Si-based peak atworking 0.38 V becomes electrode clearly (50%SiNP- visible. The50%PANI) existence (ofB) two in SiCR2023 peaks indicatesthe very that beginning with time more(Cycle+ pronounced 1), representing gradual phase high transformation reactivity takesfor placeLi-ion. In insertion this case, stainless steelinto thecoin carbon cells vs. phase Li/Li as referenceopposed toelectrode the silicon at room phase, temperature. which does not present a peak until Cycle 5 at 0.52 V. However, with the growing number of cycles, the Insecond the case anodic of PAA Si (Figurepeak at 50.38A), a V small becomes anodic clearly peak visible.at 0.25 VThecorresp existenceonds toof delithiation two Si of the carbonpeaks phase indicates and the that flow with of electronstime more into pronounced the SiNPs- Cgradual-PAA working phase transformation electrode. It is necessarytakes to emphasize, place. In thisthat case,this peak two issteps visible corresponding from the very to beginningdifferent (Cycleenergies 1) and, representing crystal high reactivity for Li-ion insertion into the carbon phase as opposed to the silicon phase, which structural modifications of silicon –lithium LixSiy alloy in the presence of PAA does not present a peak until Cycle 5 at 0.52 V. However, with the growing number of cycles, and carbon are assumed. the second anodic Si peak at 0.38 V becomes clearly visible. The existence of two Si peaks indicates thatRegarding with time the more PANI-based pronounced electrode gradual (Figurephase transformation 5B), a very different takes place electrochem. In this -case, ical picture is observed. Since the PANI-based electrode does not contain carbon, the first peak in the lower voltage range is absent. However, two silicon peaks are 260 Proceedings of the South Dakota Academy of Science, Vol. 94 (2015) present from the very beginning (Cycle 1), suggesting that a better electronic and ionic transport takes place at the interface with SiNPs in the presence of PANi, making Li exchange more favorable. The second “high energy” peak (0.48V) disappears with cycling, suggesting that the system reaches an equilibrium and that the delithiation process requires less energy consumption. Furthermore, comparison of the major silicon peaks after multiple cycles in SiNP-PAA (0.52V) and SiNP-PANI (0.33V) show that more energy is required for delithiation of PAA- vs. PANI-based electrodes. Based on our observation, the stability of the PANI-based electrodes can be explained by the improved ratio of the ionic and electronic conductivity at the interface between the PANi and SiNPs.

CONCLUSIONS

The superior performance of PANI as a conductive binder for Li-ion battery anode has been demonstrated in comparison to PAA. The effects of heat treatment were investigated for both materials, indicating that that heat treatment of PAA is detrimental to electrode performance, while the heat treatment of PANI greatly improves cycle stability. It is assumed that the heat treatment decreases the acidity of both polymer binders, therefore reducing the presence of parasitic SEI forming reactions. In comparison to the PAA, SiNP-PANi -based electrodes demonstrate the most stable cyclic stability with almost zero capacity loss between cycles. However, heat treatment negatively affected the conductivity of PAA, and ultimately decreases its performance. The results confirmed by the CV analysis indicate that PANi is superior to PAA as a binder due to its electrically conductive properties and that the electrode performance can be optimized with heat treatment.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support from National Aeronautics and Space Administration (NASA) EPSCoR program (Grant No.: NNX14AN22A) and summer scholarship program from NASA Glenn Research Center.

LITERATURE CITED

Cui, L.-F., R. Ruffo, C.K. Chan, H. Peng, and Y. Cui. 2009. Crystalline- amorphous core−shell silicon nanowires for high capacity and high current battery electrodes. Nano Letters 9:491-495. doi: 10.1021/nl8036323. Daneluti, A. L. M., and J.D.R. Matos. 2013. Study of thermal behavior of phytic acid. Brazilian Journal of Pharmaceutical Sciences 49:275-283. Proceedings of the South Dakota Academy of Science, Vol. 94 (2015) 261

Kawamura, T., S. Okada, and J.-i. Yamaki. 2006. Decomposition reaction of LiPF6-based electrolytes for lithium ion cells. Journal of Power Sources 156:547-554. doi: http://dx.doi.org/10.1016/j.jpowsour.2005.05.084. Kim, J.-M., H.-S. Park, J.-H. Park, T.-H. Kim, H.-K. Song, and S.-Y. Lee. 2014. Conducting polymer-skinned electroactive materials of lithium-ion batteries: ready for monocomponent electrodes without additional binders and conductive agents. ACS Applied Materials & Interfaces 6:12789-12797. doi: 10.1021/am502736m. Li, Z.-F., H. Zhang, Q. Liu, Y. Liu, L. Stanciu, and J. Xie. 2014. Novel pyrolyzed polyaniline-grafted silicon nanoparticles encapsulated in graphene sheets as Li-ion battery anodes. ACS Applied Materials & Interfaces 6:5996-6002. doi: 10.1021/am501239r. Lu, X., C.Y. Tan, J. Xu, and C. He. 2003. Thermal degradation of electrical conductivity of polyacrylic acid doped polyaniline: effect of molecular weight of the dopants. Synthetic Metals 138:429-440. doi: http://dx.doi.org/10.1016/ S0379-6779(02)00471-X. Magasinski, A., B. Zdyrko, I. Kovalenko, B. Hertzberg, R. Burtovyy, C.F. Huebner, and G. Yushin. 2010. Toward efficient binders for Li-ion battery Si-based anodes: Polyacrylic acid. ACS Applied Materials & Interfaces 2:3004-3010. doi: 10.1021/am100871y. Pan, L., G. Yu, D. Zhai, H.R. Lee, W. Zhao, N. Liu, Z. Bao. 2012. Hierarchi- cal nanostructured conducting polymer hydrogel with high electrochemical activity. Proceedings of the National Academy of Sciences 109:9287-9292. doi: 10.1073/pnas.1202636109 Qi, Y., J. Zhang, S. Qiu, L. Sun, F. Xu, M. Zhu, and D. Sun. 2009. Thermal stability, decomposition and glass transition behavior of PANI/NiO composites. Journal of Thermal Analysis and Calorimetry 98:533-537. doi: 10.1007/s10973-009-0298-7. Tang, Q., X. Sun, J. Wu, Q. Li, and J. Lin. 2009. Design and electrical conductivity of poly(acrylic acid-g-gelatin)/graphite conducting gel. Polymer Engineering and Science 49:1871-1878. doi: 10.1002/pen.21409. Wang, B., X. Li, T. Qiu, B. Luo, J. Ning, J. Li, and L. Zhi. 2013. High volu- metric capacity silicon-based lithium battery anodes by nanoscale system engineering. Nano Letters 13:5578-5584. doi: 10.1021/nl403231v. Wu, H., G. Yu, L. Pan, N. Liu, M.T. McDowell, Z. Bao, and Y. Cui. 2013. Stable Li-ion battery anodes by in-situ polymerization of conducting hydrogel to conformally coat silicon nanoparticles. Nature Commun. 4:1-6. doi: 10.1038/ncomms2941.