Article pubs.acs.org/JPCC
Insight into the Solid Electrolyte Interphase on Si Nanowires in Lithium-Ion Battery: Chemical and Morphological Modifications upon Cycling ,† ́ Catarina Pereira-Nabais,† Jolanta Swiatowska,* Alexandre Chagnes,† Aurélien Gohier,‡,§ Sandrine Zanna,† Antoine Seyeux,† Pierre Tran-Van,§ Costel-Sorin Cojocaru,‡ Michel Cassir,† and Philippe Marcus† †
Institut de Recherche de Chimie Paris, CNRS − Chimie ParisTech (UMR 8247), 11 rue Pierre et Marie Curie, 75005 Paris, France Laboratoire de Physique des Interfaces et Couches Minces, CNRS (UMR 7647), École Polytechnique, Route de Saclay, 91128 Palaiseau, France § Renault, Electric Storage System Division, 1 avenue du Golf, 78288 Guyancourt, France ‡
ABSTRACT: The effect of lithiation−delithiation rate and of the number of cycles on the properties of Si nanowires (SiNWs) and electrolyte interface is presented in this paper. The surface and bulk modifications of SiNW electrode induced by electrochemical process of lithiation−delithiation were investigated by combined electrochemical tests (galvanostatic cycling), field emission gun scanning electron microscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry. Low lithiation−delithiation rate improves electrochemical performance due to a better penetration depth of lithium into the SiNW electrode and the formation of a homogeneous solid electrolyte interphase (SEI) layer on the SiNWs after the first cycle. However, after repeated cycling, SiNWs suffered strong mechanical stress leading to a rough or porous SiNW structure covered by a porous SEI layer. This study highlights the SEI modifications caused by the lithiation−delithiation rate and the modifications of the Si electrode upon cycling.
1. INTRODUCTION Recent research on anodes for lithium-ion batteries has been mainly focused on silicon (Si) because of its high theoretical capacity (3579 mAh/g), low voltage plateau (0.1 V vs Li/Li+), nontoxicity, low cost, and high abundance. However, Si suffers from large volume variation (+270%)1 during lithiation which causes particle pulverisation2 and amorphization,3,4 loss of electrical contact, and early cycling capacity fading. Si capacity loss models have been proposed to understand Si particle fracture.5 Moreover, the capacity retention depends also on formation of a stable solid electrolyte interphase (SEI) layer on the Si electrode, i.e., which should fully cover the electrode surface and has a homogeneous chemical distribution with good electronic and ionic conductivity properties. However, because of huge volume variation of Si through the lithiation− delithiation process and exposition of new Si surfaces to the electrolyte, the SEI layer is not stable and reconstitutes during each lithiation process. To overcome mechanical stresses experienced by Si electrodes and to maintain their initial architecture, several strategies are used: optimization and selection of inorganic6 and organic7 binders and addition of carbon conductive matrix,8 introduction of additional transition metals inactive in alloying process (volume buffer)9 or Si amorphous phases,10,11 and finally application of nanostructured materials (like Si nanowires12−16). Particularly interesting are Si nanostructured materials which show better cycling © 2014 American Chemical Society
performances due to the increase of surface/volume ratio. Si nanowires have been improved by using a core−shell structure,17,18 i.e., C@Si,19 Si@C,20 Cu@Si,21 or
[email protected] Considering the big volume variations of Si electrodes upon cycling, understanding the electrolyte degradation mechanisms is crucial for improvement of Si electrode performances. Several studies performed on the Si-based electrode used powerful techniques, like X-ray photoelectron spectroscopy (XPS),23,24 time-of-flight secondary ion mass spectrometry (ToF-SIMS),24 Fourier transform infrared spectroscopy (FTIR),25 and solidstate high-resolution nuclear magnetic resonance (NMR)26 to characterize interfacial reaction products. SEI layer morphology24,27 and Si cracking28 were characterized by scanning electron microscopy (SEM) and optical microscopy, respectively. Electrolyte reduction mechanisms, first proposed for graphite, were rapidly extended to Si electrodes.23,24 Most of the Si electrodes were cycled in alkyl carbonates electrolytes containing 1 M LiPF6, whereas few studies used 1 M LiClO4.24 LiPF6 salt degradation can lead to the formation of fluoride species, like LiF, LixPOyFz, and SixOyFz in the SEI layer. The formation of LixPOyFz species is due to the thermal instability of LiPF6 and its reaction with alkyl carbonates. SixOyFz, Received: October 1, 2013 Revised: January 16, 2014 Published: January 22, 2014 2919
dx.doi.org/10.1021/jp409762m | J. Phys. Chem. C 2014, 118, 2919−2928
The Journal of Physical Chemistry C
Article
battery grade, Sigma-Aldrich). Water content in electrolyte (30 ppm) was determined by a Karl Fischer coulometer. A lithium foil (99.9% purity, Aldrich) was used as counter and reference electrodes. The cells were assembled in a glovebox (O2 ∼ 1 ppm; H2O < 3 ppm). Galvanostatic curves were acquired by a VMP3 Biologic multichannel potentiostat−galvanostat equipped with EC-Lab software for data acquisition. Current densities of 0.5 and 1.2 mA/cm2 (corresponding to 1.9 and 4.5 μA/μg, respectively) were applied to SiNWs having a mass density of 260 μg/cm2. The first lithiation was run from open circuit voltage (OCV) to 20 mV, while the next lithiations were limited to 120 mV versus Li/Li+ in most of the experiments. The limitation of lithiation to 20 and 120 mV during the first and the following cycles, respectively, was necessary for better capacity retention.14 Cycled SiNW-samples were washed with DMC and dried with Ar flow before SEM, XPS, and ToF-SIMS analyses. Topographic characterization of the SiNWs (after cycling) was done using a field-emission gun scanning electron microscope (FEG-SEM) Gemini LEO 1530. X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical composition of the SiNW (before and after cycling), using the Thermo Scientific ESCALAB250 spectrometer equipped with an Al Kα monochromatised radiation (hν = 1486.6 eV) and directly connected to a glovebox.24 The base pressure in the analysis chamber was ∼1 × 10−9 mbar. Survey scans were recorded with a pass energy of 100 eV, while highresolution spectra (C1s, F1s, and Si2p), presented here, were acquired with a pass energy of 20 eV. All the spectra were acquired and fitted with the Thermo Avantage v.3.13 software. For peak decomposition and quantification analysis a Shirley background subtraction, a Gaussian/Lorentzian (70%/30%) distribution, and Scofield cross-section factors were used.48 An IONTOF ToF-SIMS 5 spectrometer, directly coupled to a glovebox, was used in the dual beam analysis mode to collect negative ion depth profiles of pristine and cycled SiNWs. For sputtering, a 500 eV Cs+ beam with a 30 nA current was rastered on a 300 × 300 μm2 area, whereas for analysis, a pulsed 25 keV Bi+ beam with a 1.4 pA current and 100 × 100 μm2 area centered at the bottom of the crater was used. Both beams are at an angle of 45° with sample. The spectrometer was run at an operating pressure of 10−9 mbar. Data acquisition and processing were done using the Ion-Spec software.
originating mainly from reaction of HF with SiO2 was shown to have a deleterious effect on Si electrode performance.29 Water content in electrolytes containing LiPF6 was considered as an important parameter influencing the cycling performance of the Si electrode because of the formation of HF and of undesired products. The SEI composition can vary as a function of Si phases (amorphous30 versuss crystalline15), thin films versus nanowires,15 state of lithiation,15,23,25,31,32 number of cycles,33 and Si doping (p or n-type).34 It was reported that the SEI formed on amorphous Si during lithiation is composed of organic products originating from solvent degradation, while during delithiation the SEI contains inorganic decomposition products.35 Previous studies showed that the stability of the SEI layer on the Si electrode can be controlled either by Si surface grafting36−38 or by addition of inorganic39 or organic31,40−42 additives to the electrolyte. These additives can also improve the thermal stability of SEI layers, as previously shown for graphite anodes43−45 and compiled in a review by Zhang.46 Elimination of SEI electrochemical formation (irreversible capacity loss) has been proposed by a direct contact between Si electrode and Li metal.47 In a previous work24 on amorphous Si thin film electrodes (a-Si:H) and SiNWs in propylene carbonate (PC)/LiClO4 1M, performed by XPS and ToF-SIMS, we have shown that lithium can be trapped in the bulk of electrode material. ToF-SIMS depth profile of a-Si:H cycled electrode showed increased accumulation of Li in presence of SiO2, which may indicate an enhanced electrode reactivity in the presence of oxygenated species. It was also shown that a thicker SEI layer is formed on the Si electrode cycled in PC/LiClO4 1 M electrolyte with respect to EC:DMC (1:1)/LiPF6 1M. The NMR postmortem analysis of Si composite electrodes, performed by Oumellad et al.,26 showed that a poor cycling performance of the Si electrode can be attributed to the formation of thick SEI layer and to lithium trapping. Thus, our previous study24 was dedicated to comparative studies of electrochemical behavior of a silicon thin film model electrode and a silicon nanowire film-electrode. Analysis of both Si electrode geometries (thin film and nanowires) was necessary for validating the analytical tools used for the characterization of chemical composition and morphology of the SEI layer. The aim of the present work is to understand the dynamic behavior of SiNW electrode during lithiation−delithiation multiple cycling. This study is focused on the formation and evolution of the SEI upon cycling (up to 50 cycles) under two different current densities of lithiation: 0.5 and 1.2 mA/cm2. The electrochemical tests performed by means of galvanostatic cycling are combined with morphological (SEM) and surface chemical analyses (XPS and ToF-SIMS). These thorough analyses of the Si nanostructured electrodes presented in this work also indicate the impact of lithiation−delithiation rate on the Si electrode and its SEI morphology. The presented data should allow the better understanding of modification of the SEI layer properties as a function of the lithiation rate.
3. RESULTS AND DISCUSSION 3.1. Galvanostatic Characterization of SiNWs. The first and second galvanostatic curves of SiNWs obtained by applying a current density of 0.5 mA/cm2 are presented in Figure 1. First lithiation was performed with a cutoff of 20 mV versus Li/Li+. At potentials lower than 1.5 V (region A), water trace reduction products and SEI layer products are formed, while at potentials lower than 0.13 V (region B), a single flat plateau corresponding to lithiation of crystalline SiNW (c-SiNW) and formation of an amorphous lithiated phase (a-LixSiNW) is observed. During the first delithiation, two slopping plateaus at around 0.28 and 0.46 V (regions C and D, respectively) appear. These regions are attributed to high- and low-voltage delithiation of a-LixSiNW and formation of amorphous silicon (Table 1).16 The second lithiation was performed at two different cutoff voltages: 20 mV (solid line) and 120 mV (dashed line). In the second cycle and for both cutoff voltages, one may notice a decrease in the A region length and the appearance of a new
2. EXPERIMENTAL SECTION Si nanowires (SiNW preparation procedure described elsewhere24) were used as working electrodes in Teflon Swagelok half-cells (SiNW/Li metal) using Celgard 2100 separator, ethylene carbonate (EC, 99% purity, Alfa Aesar), and dimethyl carbonate (DMC, ≥ 99% purity, Sigma-Aldrich) [(EC:DMC) = (1:1) (wt:wt)] containing 1 M LiPF6 (purity >99.99%, 2920
dx.doi.org/10.1021/jp409762m | J. Phys. Chem. C 2014, 118, 2919−2928
The Journal of Physical Chemistry C
Article
sponding to the outermost part of the SiNW) is lithiated and that the crystalline Si core of the SiNW is preserved. Repeated lithiation can lead to complete amorphization of the c-Si core structure after a few cycles. As Si amorphization is responsible for poor electrical conduction and low capacity retention upon cycling, complete SiNW amorphization should be avoided.14 Reactions taking place during the lithiation−delithiation process in SiNWs are summarized in Table 1. Figure 2 displays the effect of the applied current density (0.5 and 1.2 mA/cm2, solid and dashed line, respectively) on the
Figure 1. First and second galvanostatic profiles of Li/c-SiNW cells in EC:DMC (1:1) (wt:wt)/LiPF6 (1M) at 0.5 mA/cm2 with a lowvoltage cutoff: 20 mV for both cycles (solid blue line); 20 (first cycle) and 120 mV (second cycle) (dashed red line).
plateau at 0.24 V corresponding to region B1 (see Figure 1). Region B1 is attributed to the lithiation of amorphous SiNW (aSiNW) formed during the first cycle. Moreover, in the second cycle a shorter plateau corresponding to lithiation of crystalline SiNW is observed (labeled region B2). After the first cycle, a aSi/c-Si shell/core-like structure is probably formed because the length of the plateau corresponding to crystalline silicon is shorter (region B2 in the second cycle is shorter than region B of the first cycle) but still preserved. Previous CV results on SiNWs already showed that the intensity of the reduction peak at ∼0.2 V associated with lithiation of amorphous silicon increased with the number of cycles, while the reduction peak at ∼0.1 V associated with the lithiation of crystalline silicon decreased.24 For SiNWs lithiated up to 20 mV (solid line) during the second cycle, a two-plateau region is also observed with potential values of 0.28 and 0.46 V during delithiation (regions C and D of the second cycle in Figure 1) as in the first cycle. For SiNWs lithiated only up to 120 mV (dashed line) during the second cycle, a single anodic process is observed (region E of the second cycle in Figure 1). This indicates that at potentials higher than 120 mV, only amorphous Si (corre-
Figure 2. Galvanostatic profiles of Li/c-SiNW cells in EC:DMC (1:1) (wt:wt)/LiPF6 (1M) at 0.5 mA/cm2 (solid line) and 1.2 mA/cm2 (dashed line) with a low-voltage cutoff: 20 mV (first cycle) and 120 mV (next cycles), respectively. Time scale of SiNWs cycled to 1.2 mA/ cm2 was magnified 12.5 times.
galvanostatic profiles. Time scale of the SiNWs cycled to 1.2 mA/cm2 was magnified 12.5 times to allow a better comparison with SiNWs cycled at 0.5 mA/cm2. The same shape of discharge curves corresponding to lithiation of SiNWs indicates the similar electrochemical processes, independent of applied current density. However, during delithiation, the value of the applied current density seems to have an impact on the curve profile and thus on the electrochemical processes. For SiNWs cycled with a density current of 1.2 mA/cm2 (dashed line in Figure 2), a single flat plateau characteristic of a two-phase region is observed. This behavior is observed for SiNW cycled
Table 1. Electrochemical Reactions Associated with Electrochemical Processes Given in Figure 1 capital letters of Figure 1 A B C D A B1 B2 C D A B1 E
electrochemical reactions first cycle (20 and 120 mV cuttoff) SEI formation (
