Eﬀect of Lithiation Potential and Cycling on Chemical and Morphological Evolution of Si Thin Film Electrode Studied by ToF- SIMS Catarina Pereira-Nabais, † Jolanta S ́ wiatowska,* ,† Michel Rosso, ‡ Franc ̧ ois Ozanam, ‡ Antoine Seyeux, † Aure ́ lien Gohier, § Pierre Tran-Van, § Michel Cassir, † and Philippe Marcus † † Institut de Recherche de Chimie Paris, CNRSChimie ParisTech, 11 rue Pierre et Marie Curie, 75005 Paris, France ‡ Laboratoire de Physique de la Matie ̀ re Condense ́ e, CNRS (UMR 7643), E ́ cole Polytechnique, 91128 Palaiseau, France § Renault, Electric Storage System Division, 1 avenue du Golf, 78288 Guyancourt, France ABSTRACT: 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 (D Li ) 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 SiO 2 quantity. These modiﬁcations can result in limitation the ionic transport of Li. KEYWORDS: Si thin ﬁlm electrode, SEI layer, ToF-SIMS, volume expansion/shrinkage, Li trapping, diﬀusion 1. INTRODUCTION Very high charge capacity (3579 mAh/g) of Si negative electrode in Li-ion batteries is related to the formation of Li- rich alloys. However, the lithium alloying/dealloying reaction leads to huge volume variation and high mechanical stresses that are responsible for electrode cracking and crumbling which can trigger poor cycling ability. 1−3 It has been reported recently that a poor cyclability of Si electrodes was associated with lithium segregation 4,5 and induced stresses 6 at the electrode/ current collector interface. The volume variations of Si electrode and resulting electrode cracking entail a continuous growth of solid electrolyte interphase (SEI) layer on the newly exposed surface of Si electrode. 7,8 Thus, taking into consideration these modiﬁcations, the formation of the SEI layer is not only limited to the ﬁrst lithiation/delithiation cycle but it continues in the following cycles. It should be also emphasized that the morphology and composition of the SEI layer strongly depend on the electrolyte composition 9−15 and state of lithiation. 16−19 In this Research Article, the composition of the SEI layer, and the chemical and the morphological modiﬁcations of the Si (a-Si:H) thin ﬁlm electrode were studied as a function of lithiation potential and number of cycles by means of time-of- ﬂight secondary ion mass spectrometry (ToF-SIMS) depth proﬁles. ToF-SIMS is a surface sensitive technique allowing for local ions detection (i.e., lithium ions and species characteristic of the products of electrolyte decomposition and analyzed sample) with a high sensitivity and a very high in-depth resolution (∼1 nm). 7,8 Moreover, changes in the in-depth ions intensities and modiﬁcations in time of sputtering induced either by lithiation potential or cycling allow an easy discrimination between SEI layer zone, Si thin ﬁlm electrode zone and stainless steel (SS) current collector zone. The application of the Si model thin ﬁlm electrode having enlarged surface-to-volume ratio, provides clear and more comprehen- sible insight into the electrode/electrolyte interface reactions without complications from current percolators or binding agents 8 that are used in bulk composite electrode materials. This allows us to study intrinsic electrochemical and interfacial processes occurring on the surface of electrode materials. ToF- SIMS depth proﬁle is also used for the direct calculation of the apparent diﬀusion coeﬃcient of Li ions (D Li ) in 100 nm Si ﬁlms using the semi-inﬁnite integration of Fick’s second law for one- dimensional diﬀusion. Received: May 13, 2014 Accepted: July 24, 2014 Published: July 24, 2014 Research Article www.acsami.org © 2014 American Chemical Society 13023 dx.doi.org/10.1021/am502913q | ACS Appl. Mater. Interfaces 2014, 6, 13023−13033