a b s t r a c tThe effect of the Si electrode morphology (amorphous hydrogenated silicon thin films -a-Si:H as a model electrode and Si nanowires -SiNWs electrode) on the interphase chemistry was thoroughly investigated by the surface science techniques: X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). XPS analysis shows a strong attenuation and positive shift of the Si 2p peaks after a complete charge/discharge performed in PC-and EC:DMC-based electrolytes for both electrodes (a-Si:H and SiNW), confirming a formation of a passive film (called solid electrolyte interphase -SEI layer). As evidenced from the XPS analysis performed on the model electrode, the thicker SEI layer was formed after cycling in PC-based electrolyte as compared to EC:DMC electrolyte. XPS and ToF-SIMS investigations reveal the presence of organic carbonate species on the outer surface and inorganic salt decomposition species in the inner part of the SEI layer. Significant modification of the surface morphology for the both electrodes and a full surface coverage by the SEI layer was confirmed by the scanning electron microscopy (SEM) analysis.
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.
Si thin films obtained by plasma enhanced chemical vapor deposition (PECVD) were used to investigate chemical and morphological modifications induced by lithiation potential and cycling. These modifications were thoughtfully analyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiling, which allows to distinguish the surface and bulk processes related to the formation of the solid electrolyte interphase (SEI) layer, and Li-Si alloying, respectively. The main results are a volume expansion/shrinkage and a dynamic behavior of the SEI layer during the single lithiation/delithiation process and multicycling. Trapping of lithium and other ions corresponding to products of electrolyte decomposition are the major reasons of electrode modifications. It is shown that the SEI layer contributes to 60% of the total volume variation of Si electrodes (100 nm). The apparent diffusion coefficient of lithium (DLi) calculated from the Fick's second law directly from Li-ion ToF-SIMS profiles is of the order of ∼5.9 × 10(-15) cm(2).s(-1). This quite low value can be explained by Li trapping in the bulk of electrode material, at the interfaces, continuous growth of the SEI layer and increase of SiO2 quantity. These modifications can result in limitation the ionic transport of Li.
Epoxy-amines are used as structural adhesives deposited on Ti. The amine adhesion to a Ti surface depends highly on the surface state (oxidation, hydroxylation). Amines may adsorb above preadsorbed water molecules or substitute them to bind directly to surface Ti(4+) Lewis acid sites. The adsorption of a model amine molecule, diaminoethane (DAE), on a model surface, hydrated TiO2-anatase (101) surface, is investigated using Density Functional Theory including Dispersive forces (DFT-D) calculations. DAE adsorption and water substitution by DAE are exothermic processes and turn nearly isoenergetic at high coverage with adsorption-substitution energies around -0.3 eV (including dispersion forces and ZPE). Complementary ab initio molecular dynamics studies also suggest that the formation of an amine-water interaction induces water desorption from the surface at room temperature, a preliminary step towards the amine-Ti bond formation. An atomistic thermodynamic approach is developed to evaluate the interfacial free energy balance of both processes (adsorption and substitution). The main contributions to the energetic balance are dispersive interactions between molecules and the surface on the exergonic side, translational and rotational entropic contributions on the endergonic one. The substitution process is stabilized by 0.55 eV versus the adsorption one when free solvation, rotational and vibrational energies are considered. The main contribution to this free energy gain is due to water solvation. The calculations suggest that in toluene solvent with a water concentration of 10(-4) M or less, a full DAE layer replaces a preadsorbed water layer for a threshold concentration of DAE ≥ 0.1 M.
In situ electrochemical (by electrochemical impedance spectroscopy, EIS) and ex situ surface (by time-of-flight secondary ions mass spectrometry, ToF-SIMS) analysis were applied to investigate solid-state diffusion coefficient (D Li ) into conversion-type α-Fe 2 O 3 negative electrode for Li-ion batteries. D Li values obtained from EIS were in the range of 10 −16 to 10 −15 cm 2 s −1 for electrodes partially and fully lithiated, respectively, showing that pulverization of the converted material promotes Li-ion migration. ToF-SIMS ion depth profiling performed after partial lithiation enabled discriminating the surface solid electrolyte interphase (SEI) region, a converted electrode region (Li 2 O/Fe 0 matrix) of slow diffusion (D Li = 6 × 10 −16 cm 2 s −1 ) and an unconverted region (intercalated Fe 2 O 3 matrix) of faster diffusion (D Li = 2 × 10 −13 cm 2 s −1 ) ahead of the conversion front. Comparison of the ex situ and in situ results indicates that the electrode conversion kinetics is limited by Li-ion diffusion in the converted matrix and suggests a hindering effect of the passivating SEI layer. ToF-SIMS depth profile analysis appears as a most appropriate and direct methodology to measure Li-ion diffusion solely in electrode materials, excluding SEI layer effects.
Understanding the electrode processes occurring at the electrode/electrolyte interface and in the bulk electrode material is necessary for developing the electrochemical performances of lithium-ion (LIB), sodium-ion, sulfur or metal-air batteries. The main electrode processes are insertion/extraction reactions that induce changes in the host electrode materials; they are accompanied by decomposition of electrolyte that leads to formation of a solid electrolyte interphase (SEI) layer [1]. Apart from the electrode processes, the electrochemical performance of batteries can also be significantly dependent on ionic transport, thus a good understanding of diffusion processes is also necessary. Two principal surface-sensitive techniques are particularly suitable for analyzing surface reactions at electrodes: X–ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). As XPS provides information on chemical composition on the first few nanometers at the electrode surface, depth profiling analysis is also necessary for the characterization of thicker surface layers and bulk materials. Depth profiling can be either performed by XPS or ToF–SIMS using Ar+ or Cs+ ions, respectively, for sputtering. ToF-SIMS is a highly sensitive surface analytical technique where a pulsed primary ion beam (e.g. Bi+) is used to extract secondary ions that are analyzed by time–of–flight spectrometry. Interlaced with a sputtering ion beam (e.g. Cs+), elemental depth profiles with excellent depth resolution (monolayer) and high sensitivity (ppb) can be readily obtained. ToF-SIMS can also serve as a new and more direct method than the commonly used electrochemical methods (i.e. electrochemical impedance spectroscopy, cyclic voltammetry or galvanostatic methods) for precise measurement of ion diffusion in electrode materials. Our approach is to use model thin-film electrodes (i.e. from few tens to few hundreds of nm directly grown on a current collector) with a good surface finishing and having a roughness suitable for analysis by surface sensitive techniques. The application of model thin-film electrode having enlarged surface-to-volume ratio provides clearer and more comprehensible insight into electrode/electrolyte interface reactions without complications from current percolators or binding agents that are used in bulk composite electrode materials. This allows for studies of intrinsic electrochemical and interfacial processes occurring on the electrode surfaces. In the present work, XPS and ToF-SIMS have been used to investigate the chemical and volume modifications of thin-film negative-electrode materials. As an example, two types of high capacity negative-electrode materials will be presented: a conversion-type electrode consisting of iron oxide (with a theoretical capacity of 1007 mAh/g), and an alloying-type electrode consisting of silicon (with a theoretical capacity of 3579 mAh/g). Both negative-electrode materials are interesting and important candidates for application in LiBs due to their abundance and environmental friendliness. For the first time, ToF-SIMS depth profiling have been applied to measure an apparent diffusion coefficient of lithium ions (DLi+) into these electrodes. The DLi+was calculated from the finite integration of Fick’s second law for one-dimensional diffusion. Regardless of the type of electrode material, XPS analysis shows strong intensity attenuation and binding energy positive shift of the principal core level peak corresponding to the main electrode component (Si2p in the case of Si-electrode and Fe2p in the case of Fe2O3-one), confirming the formation of the SEI layers on the surfaces after cycling in PC-LiClO4. The chemical composition of the SEI formed on the two different electrodes is essentially identical, with a principal Li2CO3 constituent and a minor quantity of alkyl carbonates (ROCO2Li) as evidenced by the C1s, O1s and Li1s core level peaks [2,3]. The SEI layer chemistry principally depends on the electrolyte composition [2]. Both, XPS and ToF-SIMS, evidence a dynamic increase/decrease of the SEI layer thickness upon lithiation/delithiation, superimposed to a continuous increase upon cycling. The data reveal the formation of duplex-like SEI layer structure, with the organic carbonate species in the outer region and inorganic salt decomposition products in the inner part of the SEI layer. The ToF-SIMS depth profiles also show irreversible Li trapping in the bulk electrodes and volume variations with the degree of lithiation and the number of cycles [2,4]. These modifications can be detrimental to electrode cycling and capacity retention. A low DLi+ in the order of 10-15 cm2/s calculated from ToF-SIMS depth profiles for these two types of electrodes, indicates that the rate of lithiation is slower than for intercalation-type materials and can have an important influence on their electrochemical performance. References [1] E.Peled, J.Electrochem.Soc.126(1979)2047. [2] C.Pereira-Nabais, J.Światowska, A.Chagnes, F.Ozanam, A.Gohier, P.Tran-Van, C.–S.Cojocaru, M.Cassir, P.Marcus, App.Surf.Sci.266(2013)5. [3] B.Tian, J.Światowska, V.Maurice, S.Zanna, A.Seyeux, L.H.Klein, P.Marcus, J.Phys.Chem. C117(2013)21651. [4] B.Tian, J.Światowska, V.Maurice, S.Zanna, A.Seyeux, L.H.Klein, P.Marcus, Langmuir (2014), DOI:10.1021/la404525v.
Silicon is considered as a promising anode material for Li-ion batteries due to its ability to insert large amounts of lithium, delivering a very high theoretical specific capacity of 3579 mAh/g, which is almost 10 times higher than graphite electrode (372 mAh/g). Nevertheless, a high volume variation (280 % for Li15Si4) during lithiation leading to a morphological damage of electrode materials and a huge capacity loss (of about 30%) observed during the first charge are the major drawbacks for application of Si as anode material. These damages can be considerably decreased or avoided by using nanosized materials, such as Si nanowires (SiNW), allowing better accommodation of volume variation.[i],[ii] Another solution to limit the consequences of these damages is to use electrolyte additives like vinylene carbonate (VC) and monofluoroethylene carbonate (FEC), having polymerizable features and the possibility of forming a SEI layer with improved mechanical properties.[iii],[iv] A new way to improve the Si electrode performances is the modification of the chemical bulk composition of the Si electrode material. In this work, we present the improved electrochemical performance of methylated amorphous silicon (a-Si0.9(CH3)0.1:H), a new type of Si-based material containing methyl (CH3) groups. In order to focus on the material itself, we compare thin-film electrodes of this material[v] to similar electrodes of hydrogenated amorphous Si (a-Si:H),[vi] without using additives or nanosize shaping of the material. The chemical modifications of these two electrodes induced by the electrochemical lithiation process were studied by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) negative-ion depth profiles. Experiments were performed in propylene carbonate (PC, purity > 99.7 %, 30 ppm H2O, Sigma-Aldrich) containing 1 M LiClO4 (purity > 99.99%, battery grade, Sigma-Aldrich). As compared to a-Si:H, the modified chemical composition of a-Si0.9(CH3)0.1:H has an influence on the electrochemical behaviour by shifting the lithiation plateau from 200 mV to 160 mV. The most important difference between the two types of electrodes is the instantaneous formation of a thin, stable and homogenous Solid Electrolyte Interphase (SEI) layer on a-Si0.9(CH3)0.1:H electrode during the first cycle. On the contrary, a thick unstable SEI layer is formed on a-Si:H, with possible dissolution/oxidation and cracking of the layer formed during the first cycle and continuous uptake of electrolyte decomposition products during the following discharge/charge cycles. The ion-depth profiles obtained by ToF-SIMS also evidence significant volume variations and swelling of a-Si:H electrodes which can lead to large morphological modifications, electrode cracking. The a-Si0.9(CH3)0.1:H electrode also shows much lower irreversible capacity during the first 100 discharge/charge cycles, as compared to the a-Si:H electrode. Finally, the diffusion coefficient of Li ions (determined from Li- ToF-SIMS profile) is found to be one order of magnitude higher in a-Si0.9(CH3)0.1:H than in a-Si:H, confirming its improved electrochemical performance. References [i] C. K. Chan, H. Peng, G. Liu, K. McIlwrath, X. F. Zang, Y. Cui, Nat. Nanotechnol. 3, 31 (2008). [ii] B. Laïk, D. Ung, A. Caillard, C.-S. Cojocaru, D. Pribat, J.-P. Pereira-Ramos, J. Solid State Electrochem. 14, 1835 (2010). [iii] M. Ulldemolins, F. Le Cras, B. Pecquenard, V. P. Phan, L. Martin, H. Martinez, J. Power Sources 206, 245 (2012). [iv] V. Etacheri, O. Haik, Y. Goffer, G. A. Roberts, I. C. Stefan, R. Fasching, D. Aurbach, Langmuir 28, 965 (2012). [v] L. Touahir, A. Cheriet, D. Alves Dalla Corte, J.-N. Chazalviel, C. Henry de Villeneuve, F. Ozanam, I. Solomon, A. Keffous, N. Gabouze, M. Rosso, J. Power Sources 240, 551 (2013). [vi] C. Pereira-Nabais, J. Światowska, A. Chagnes, F. Ozanam, A. Gohier, P. Tran-Van, C.–S. Cojocaru, M. Cassir, P. Marcus, App. Surf. Sci. 266, 5 (2013).
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