Chem. Mater. 2010, 22, 1229–1241 1229 DOI:10.1021/cm902688w
Key Parameters Governing the Reversibility of Si/Carbon/CMC Electrodes for Li-Ion Batteries† J.-S. Bridel,‡,^ T. Azaı¨ s,§ M. Morcrette,‡,^ J.-M. Tarascon,‡,^ and D. Larcher*,‡,^ ‡
Laboratoire de R eactivit e et Chimie des Solides Universit e de Picardie Jules Verne, CNRS UMR6007 33 rue Saint Leu 80039 Amiens, France, §Universit e Pierre et Marie Curie-Paris 6 and CNRS, UMR 7574 Laboratoire de Chimie de la Mati ere Condens ee de Paris, Coll ege de France F-75005 Paris, France, and ^ ALISTORE-ERI European Research Institute Received August 31, 2009. Revised Manuscript Received November 2, 2009
Various Si/carbon/polymer composite electrodes were prepared to better understand the influence of the Si-polymer interactions on the stability of the Li-Si reaction and especially the superior performances of CMC-based (carboxy-methyl-cellulose) composites despite the large volume changes of the Si particles upon cycling. Via the modification of the composites formulation, the nature of the polymer, the nature and the amount of the substituting groups and the surface chemistry of the Si particles, together with the use of various characterization techniques (TEM, SEM, NMR-MAS, infrared spectroscopy, TGA, etc.) we could propose that the performances of the Si/Csp/CMC composite electrodes are nested in both the porous texture of the electrode and in the nature of the Si-polymer chemical bonding. A self-healing process of the rather strong Si-CMC hydrogen bonding which can accommodate textural stresses and can evolve during cycling is proposed to be critical for Si-based electrode performances. This better understanding leads to the design of Si-based electrodes with capacity retention reaching 1000 mAh/g of composite (i.e., full Si capacity) for at least 100 cycles and with a Coulombic efficiency close to 99.9% per cycle. Owing to these new aspects, we have now a deeper insight of the specific effects of the CMC binder, than could be successfully extended to other metals (Sn, Ge, Sb). Introduction One of the major challenges of our modern society is the integration of reliable energy storage systems in the upcoming global energetic schemas which are being drawn for the use of the intrinsically diffuse/intermittent renewable energy sources as alternatives to fossil fuels. Among the various available storage technologies, the electro-chemical conversion is one of the most efficient, it is easily sizable, easily adaptable to various fields (automotive, portable tools, medicine, space, electronics, etc.), and the lithium-ion cells are now the most widely used secondary batteries because of their highest energy density, high operating voltages, and low self-discharges,1,2 albeit their gravimetric and volumetric energy densities need significant improvements. Commercialized by Sony in 1990, Li-ion cells have mainly relied on the use of carbonaceous materials (372 mAh/g) at the negative electrode and layered LiCoO2 (135 mAh/g) and derivatives Li(Co,M)O23-6 M = Ni and/or Mn and/or Al, etc. ) with higher capacities † Accepted as part of the 2010 “Materials Chemistry of Energy Conversion Special Issue”.
(1) (2) (3) (4)
Tarascon, J. M.; Armand, M. Nature 2001, 404, 359. Tarascon, J. M.; Armand, M. Nature 2008, 451(7179), 652–657. Liu, Z.; Yu, A.; Lee, J. Y. J. Power Sources 1999, 81-82, 416. Rossouw, M. H.; de Kock, A.; de Piciotto, L. A.; Thackeray, M.; David, W. I. F. Master. Res Bull. 1990, 25, 173. (5) Lu, Z.; MacNeil, D. D.; Dahn, J. R. Electrochem. Solid-State Lett. 2001, 4, A200. (6) Cushing, B. L.; Goodenough, J. B. Solid State Sci. 2002, 4, 1487. r 2009 American Chemical Society
(160-200 mAh/g) or 3D spinel (LiMn2-xMxO4 with M = Ni, Co, Cr, etc. (148 mAh/g) as positive electrode materials. Nevertheless, recent findings have definitively put nanomaterials on the stage with fast implementations in commercial cells, both at the positive side with intercalation compounds (olivine LiFePO4,7 A123 Systems, 2006) and at the anode side with alloying reactions (Sn-Co-C, Sony, Nexelion, 2005). This recent revolution due to the exploration of this new degree of freedom besides structure and composition is now greatly altering the way we elaborate, characterize, and formulate the electrode materials, with mainly a renewed interest in the alloying reactions and subsequent surface activity. Experimental works on the electrochemical alloying of elements with lithium started in the early 70s when Dey8 described the formation of Li alloys at room temperature with Sn, Pb, Al, Au, etc. and pointed out that they were similar to those prepared by metallurgical processes. In 1976, Li was found to electrochemically react with Si at high temperatures9 through the successive formation of Li12Si7, Li14Si6, Li13Si4 and Li22Si510 phases, and at room (7) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144, 1188. (8) Dey, A. N. J. Electrochem. Soc. 1971, 118, 1547. (9) Sharma, R. A.; Seefurth, R. N. J. Electrochem. Soc. 1976, 123, 1763. (10) Boukamp, B. A.; Lesh, G. C.; Huggins, R. A. J. Electrochem. Soc. 1981, 128, 725.
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temperature as well.11 Even if we are now aware that this electrochemical reaction is limited at room temperature to the Li15Si4 end-member,12,13 both the corresponding gravimetric (3579 mAh/g) and volumetric (8330 mAh/ cm3) capacities are far ahead of all the other Li-uptake reactions so far identified, regardless of their nature (intercalation, alloying, conversion), and self-justify the past, present, and surely future focus on this system. However, the structural strains and above all the large volume expansion (≈275%) undertaken by the active particles during the alloying process result in particles displacement, electrode textural modification, definitive loss of electrical wiring, and finally boil-down to a very poor capacity retention.14-16 Over the last years, four main different approaches have been put forward to improve the cycle life of alloying-based electrodes, and their combinations were also tested. The first one consists of forming composite materials in which active alloy or metal particles are finely dispersed in an active or inactive solid matrix which buffers the expansion of the active phase; but this effect is not longlasting so that the capacity is solely maintained for a few tens of cycles.17-21 The second is based on the use of nanoparticles that can sustain the physical strains that they experienced along the Li uptake/removal19,22 so that the volume expansion due to the insertion of lithium causes much less cracking and particles pulverization, hence leading to better electrode integrity and improved cycling performance. The third approach relies on thin films technologies which take advantage of the constraints inherent to the grown Si films to partially maintain the electrode cohesion with the end result being an excellent capacity retention but an overall capacity limited by the back substrate dead weight.23,24 Finally, the most recent strategy deals with the utilization of selected binders that hold the integrity of the electrode by maintaining the contact between the active material (alloy/metal), the conducting additive (carbon), (11) Lai, S-C J. Electrochem. Soc. 1976, 123, 1196. (12) Obrovac, M. N.; Christensen, Leif Electrochem. Solid-State Lett. 2004, 7, 93. (13) Hatchard, T. D.; Dahn, J. R. J. Electrochem. Soc. 2004, 151(6), A838–A842. (14) Ryu, J. H.; Kim, J. W.; Sung, Y. E.; Oh, S. M. Electrochem. SolidState Lett. 2004, 7, 306–309. (15) Timmons, A.; Dahn, J. R. J. Electrochem. Soc. 2006, 153(6), A1206–A1210. (16) Huggings, R. A. J. Power Sources 1988, 22, 341. (17) Wilson, A. M.; Way, B. M.; Buuren, T. Van; Dahn, J. R. J. Appl. Phys. 1995, 77,2363. (18) Coutney, I. A.; Dahn, J. R. J. Electrochem. Soc. 1997, 144, 2045. (19) Wilson, A. M.; Dahn, J. R. J. Electrochem. Soc. 1995, 142, 326. (20) Saint, J.; Morcrette, M.; Larcher, D.; Laffont, L.; Beattie, S.; Peres, J.-P.; Talaga, D.; Couzi, M.; Tarascon, J. M. Adv. Funct. Mater. 2007, 17, 1765–1774. (21) Anani, A.; Huggins, R. A. J. Power Sources 1992, 38(3), 351. (22) Yang, J.; Winter, M.; Besanhard, J. O. Solid State Ionics 1996, 90, 281. (23) Li, H.; Huang, X.; Chen, L.; Zhou, G.; Zhang, Z.; Yu, D.; Mo, Y. J.; Pei, N. Solid State Ionics 2000, 135, 181. (24) Hatchard, T. D.; Dahn, J. R. J. Electrochem. Soc. 2004, 151, A838. (25) Chen, Z.; Christensen, L.; Dahn, J. R. Electrochem. Commun. 2003, 5, 919–923. (26) Liu, W. R.; Yang, M.; Wu, H; Chiao, S. M.; Wu, N Electrochem. Solid-State Lett. 2005, 8, A100.
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and the current collector through suited surface interaction.25,26 Note that another additional approach, deviating from the aforementioned chemical ones, proposes the setting of specific cycling procedures but will not be easily applicable in real application conditions.27 Among the multitude of polymeric molecules that can act as binders, one would naively believe that the most efficient ones will be those presenting the best elasticity and mechanical resistance to stretching. Surprisingly, SBR (styrene butadiene rubber), while having high deformation capacities,26 is not improving the cyclability in the expected extent.28 In contrast, the capacity retention of alloying processes was found to be unambiguously improved with PVDF (poly(vinylidene fluoride))-based binders, despite their intrinsic low elasticity, by chemically promoting its adhesion onto the particle25 surface. This quick literature survey obviously suggests that elasticity is not the key issue, but rather the nature of the bonding with the active particles. This was recently further pointed out by Li et al.28 whose results show that carboxyl-methyl-cellulose (CMC), while having a very important stiffness and a small elongation at break (5-8%),28 appears to be the best binder for the stability of the Li-Si reaction, to date. CMC is a linear polymeric derivative of cellulose with various degree of substitution (DS) of ;OH groups by ; CH3COO- carboxy-methyl groups (Table 1), the maximum DS being equal to 3 since each monomeric unit hangs three hydroxyl groups. As salt, the substituting groups are dissociated in water forming anionic groups responsible for the aqueous solubility of CMC in contrast with insoluble cellulose. B. Lestriez et al.29 suggest that the efficiency of CMC could be ascribed to its extended conformation in solution that facilitates the formation of an efficient network, whereas N. S. Hochgatterer et al.30 and D. Mazouzi et al.31 suggest the necessity of an ester-like Si;CH3COO; R strong covalent bond resulting from the reaction of the carboxyl groups with the ;OH groups at the surface of the thin SiO2 layer surrounding the Si particles. Besides the role of the Si/polymer interaction, the Si/C/binder proportions obviously have to be considered also in order to clearly identify the exact role of the polymer. Based on a simple geometrical model and assuming a 270% volumetric expansion (Li f Li3.75Si), S. Beattie et al.32 have shown that optimum performances are obtained for an electrode with 1/1/1 Si/C/binder weight proportions ensuring a free expansion of the particles preventing their contact hence limiting shuffling. (27) Obrovac, M. N.; Krause, L. J. J. Electrochem. Soc. 2007, 154(2), A103–A108. (28) Li, J.; Lewis, R. B.; Dahn, J. R. Electrochem. Solid-State Lett. 2007, 10(2), A17. (29) Lestriez, B.; Bahri, S.; Sandu, I.; Roue, L.; Guyomard, D. Electrochem. Commun. 2007, 9, 2801–2806. (30) Hochgatterer, N. S.; Schweiger, M. R.; Koller, S.; Raimann, P. R.; W€ ohrle, T.; Wurm, C.; Winter, M. Electrochem. Solid-State Lett. 2008, 11(5), A76–A80. (31) Mazouzi, D.; Lestriez, B.; Roue, L.; Guyomard, D. Electrochem. Solid-State Lett. 2009, 12, A215. (32) Beattie, S. D.; Larcher, D.; Morcrette, M.; Simon, B.; Tarascon, J.-M. J. Electrochem. Soc. 2008, 155, A158.
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Table 1. Molecular Structure of the Different Polymers and of the Grafted Si Samples
Within this context, the objective of this work is to conduct a global analysis of the role of the CMC polymer onto the Si/C/CMC electrode texture and electrochemical behavior. We will further show the importance of the electrode composition, and we will decipher some aspects of the polymer effects by varying its characteristics including chain length, nature, and number (DS) of the functional groups and countercation. Along that line, mindful of the wealthy literature dealing with the reactivity of the silanol group, condensation of Si-O networks and chemistry of alkoxysilanes,33 and of the easiness to modify the Si surface and modulate its reactivity and therefore the particle-polymer interaction, selected grafted groups will be also used to promote the formation of covalent bonding with the binder. Last, the nature and strength of the chemical bonding between the binder molecules and the Si surface will be shown to directly influence the electrode texture and its electrochemical performances, the characterization of our samples had to be performed through the use of techniques ranging from very local to global probing enlisting NMR-MAS, infrared spectroscopy, atomic force microscopy, and scanning electron microscopy. Experimental Techniques and Samples Descriptions Metals, Binders, and Carbon Samples. A unique batch of silicon (Aldrich) was used in this study. X-ray diffraction pattern (Figure 1a) does not show crystallized impurities, and the refined cubic cell parameter is found very close (5.4315(3) A˚) to the literature value (5.431 A˚, JCPDS no. 27-1402). Particles have spherical shape and are monodispersed with 120 nm mean size (Figure 1b). The calculated crystallite size (around 80 nm) is independent of the crystallographic direction. BET specific surface area (28 m2/g) compared to the calculated geometrical surface area (23 m2/g) revealed no significant internal particles (33) Brinker, C.J.; , Sherer, G.W., Sol-Gel Sciences: The Physics and Chemistry of Sol-Gel Processsing; Academic Press: San Diego, 1990.
Figure 1. (a) X-rays diffraction pattern, (b) TEM picture, and (c) thermogravimetric analysis (TGA) trace (air, 20/min) for the Si powder (Aldrich) used in this study.
porosity, as confirmed by direct porosity measurements. Based on the default of weight gain (Si f SiO2) measured during heating under air (Figure 1c), we evaluated the initial oxide content of this batch to be around 8 wt %. Any surface modification applied to Si particles along this work will therefore have to be considered to be applied to the capping SiO2 layer which thickness is calculated around 3 nm consistently with previous chemical and spectroscopy analysis.20 With a batch of high specific surface area silica (fumed silica, Aldrich) we verified the total absence of electrochemical reactivity of silicon oxide with lithium, as reported by numerous works. In addition, based on previous EELS (energy electron loss spectroscopy) analysis of the Si/Si;O boundary,20 we are confident in the tight core/shell bounding. Aside from Si, we also studied the reactivity of Sn, Ge, and Sb particles (Aldrich). Whatever their size (100 nm for Sn and ca. 1 μm for Ge and Sb) these powders also exhibit an oxide layer which proportion is around 6-8 wt % based on TGA analyses. Two Na-CMC (Aldrich) salts were used as received or used as precursors to synthesize other CMC batches with modified DS or exchanged countercation. Based on the data from the provider, these two polymers differ by their (MW) molecular
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weight (700 000 and 250 000 g/mol), and will thus be referred to as CMC700000 and CMC250000, respectively. As we are well aware of the difficulty of defining polymers by a unique MW value, we collected steric exclusion chromatography (SEC) data and, as expected, a large distribution in MW is observed. Despite this dispersion (from 100 000 to 6 000 000 g/mol for CMC700000, and from 10 000 to 2 000 000 g/mol for CMC250000) and deviation from the provider data (1 328 000 and 500 000 g/mol in average MW for CMC700000 and CMC250000, respectively), we could however confirm that these two batches statistically consist of polymers chains with very different length. The changes in the DS of CMC materials were conducted as described in refs 34 and 35. Typically, CMC precursor (5 g) is suspended in 200 mL of an ethanol/water (1/1 v/v) mixture, and the suspension is degassed for one hour under nitrogen flow, prior addition of few drops of acid-base indicator solution (phenolphthalein), and then dissolution of sodium hydroxide pellets (3 g). Then, 5.6 g of chloroacetic acid (Acros) is gradually added while the alkalinity of the solution is regularly maintained by adding sodium hydroxide as soon as the pink color of the indicator vanishes. During these additions, the reaction medium is maintained at 60 C, maintained at this temperature for 4 h under nitrogen flow, and then the reaction is stopped by adding glacial acetic acid. The suspension is then filtered and successively washed with ethanol/water mixtures (70/30 f 80/20 f 90/10 v/v) to fully remove impurities and excess of reagents which are harmful for capacity retention. The composition of the washing solutions prevents too much dissolution of the polymer. Each complete run averagely increases the DS value of 0.4 unit as determined by volumetric titration methods35 and infrared spectroscopy,36 the initial DS value for both CMC700000 and CMC250000 being both found equal to 0.9 (( 0.1). The maximum DS value reached that way is 1.8, and this substitution was first taking place on the primary alcohol function, which is the most accessible function, carbon usually called C6.37 To perform the counterion exchange, the acid form of CMC (H-CMC) is first synthesized by treating Na-CMC (2 g) in 100 mL of a solution of HCl (1 M) in ethanol/water (90/10 v/v) during 60 h at ambient temperature. Then, the polymer is filtered and washed with the same sequence of water/alcohol mixtures as above. Finally, by adding excess powdered hydroxide (LiOH, KOH) to the H-CMC suspension, the corresponding Li-CMC and K-CMC salts are recovered after one day reaction. No DS modification has been noticed during this exchange which yield was at least 90% as determined by EDS and atomic absorption analysis. The modification of the silicon particles surface was obtained through the reaction of the surface hydroxyl groups with 3-aminopropyl-trialkoxysilane or propyl-trialkoxysilane (Acros organics, 99%).38 The reaction of silanol (;Si;OH) groups with ;OH surface groups corresponds to a condensation with loss of water, therefore resulting in the creation of Si;O;Si covalent bridge. First, a suspension of silicon particles in toluene (10 g/L) was heated at 70 C during one hour to eliminate as much (34) Pushpamalar, V.; Langford, S. J.; Ahmad, M.; Lim, Y. Y. Carbohydr. Polym. 2006, 64, 312. (35) Miyamoto, K.; Tsuji, K.; Nakamura, T.; Tokita, M.; Komai, T. Carbohydr. Polym. 1996, 30, 161–164. (36) Barba, C.; Montane, D.; Farriol, X.; Desbrieres, J.; Rinaudo, M. Cellulose 2002, 9, 327–335. (37) Baar, A.; Kulicke, W. M. Macromol. Chem. Phys. 1994, 195, 1483– 1492. (38) Chong, A. S. M.; Zhao, X. S. J. Phys. Chem. B 2003, 107, 12650.
Bridel et al. water as possible and the silane was then added in a large excess (1 g per g of silicon). The mixture was vigorously stirred for 72 h under reflux (T = 111 C), filtered, and the recovered powder dispersed in toluene during 2 h. The solid (Si-g1 in the case of propyl-trialkoxysilane grafting; Si-g2 in the case of 3-aminopropyl-trialkoxysilane grafting) was finally collected by filtration, washed with isopropanol, and dried at room temperature under primary vacuum. Si-g2 was used as a precursor to create a covalent peptidic bond (;CO;NH;) with CMC39 (i.e., reaction between the Sig2 primary amine and the CMC carboxylic groups). The functionalized particles were suspended in a water/toluene (50/50 v/v) mixture which pH value is dropped (
