Regular Article Effects of phase assemblage and microstructure-type for Sn/intermetallic ‘composite’ films on stress developments and cyclic stability upon lithiation/delithiation Ravi Kali, Yaadhav Krishnan, Amartya Mukhopadhyay ⁎ High Temperature and Energy Materials Laboratory, Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology (IIT) Bombay, Powai, Mumbai 400076, India abstract article info Article history: Received 9 October 2016 Received in revised form 17 November 2016 Accepted 19 November 2016 Available online 30 November 2016 Annealing treatment of as-deposited β-Sn film on Cu resulted in the development of ‘composite’ film comprised of Sn-Cu intermetallic phases (Cu 3 Sn and Cu 6 Sn 5 ) surrounding ‘percolating’ network of β-Sn, all underneath a thin continuous β-Sn layer. Such phase assemblage and microstructure-type resulted in significantly improved mechanical integrity upon lithiation/delithiation; and accordingly very stable Li-capacity retention with contin- ued electrochemical cycling. In-situ monitoring of the stress developments during lithiation/delithiation indicat- ed that the presence of intermetallic ‘buffer’ phase(s) results in ~3 times lower stress magnitudes compared to ‘pure’ Sn upon ‘full’ lithiation, along with absence of signature for mechanical degradation in the stress-time profiles. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Intermetallic Microstructure In-situ stress measurement Mechanical integrity Electrochemical behavior Sn, as possible alternative to graphitic carbon as anode material for Li-ion batteries, has considerable advantages owing to the nearly three times greater specific Li-capacities (i.e., ~ 994 mAh/g for Sn, as compared to ~372 mAh/g for graphite) and improved safety aspects, especially at the higher discharge/charge rates [1–3]. However, poor cycle life due to stress induced fracture/disintegration, arising from enormous volume expansion/contraction (up to ~300%) during repeated lithiation/ delithiation cycles, is the major bottleneck towards the usage of Sn as anode material [1–5]. Among the various possible means for improving the cyclic stability of Sn-based electrodes, usages of Sn-Cu intermetallic (especially, Cu 6 Sn 5 and sometimes Cu 3 Sn, in the place of Sn) have been envisaged [6–14], where it is often believed that the ‘inactive’ Cu may act as ‘buffer’ towards the stress developments. However, direct use of such intermetallics alone do not offer consid- erable advantages since the theoretical Li-capacity of Cu 6 Sn 5 (viz., ~300 mAh/g; with ~200–250 mAh/g usually achieved) is lesser even compared to graphitic carbon, with Cu 3 Sn being interestingly reported to be ‘inactive’ against lithiation at room temperature [11,12]. Further- more, the insulating natures of such intermetallics [15] are not expected to offer any advantage towards the rate capability, as well. In this con- text, Cu 3 Sn (resistivity ~8.8 μΩ cm) fairs slightly better as compared to Cu 6 Sn 5 (resistivity ~17.5 μΩ cm) [16]. Accordingly, it is not surprising that the best reported performance to-date with such intermetallic- based electrodes, even in the case of composite with Sn (i.e., Sn/ Cu 6 Sn 5 ) [6–14], has not been more than ~450 mAh/g (delithiation ca- pacity; after 20 cycles), considering no contribution from any other ma- terial and reversibility (i.e., Coulombic Efficiency) within acceptable range. Additionally, truly significant improvement has also not been re- ported for cyclic stability, either. Against these backdrops we report here the development of Sn/Sn- Cu based ‘composite’ thin film electrodes, having desired microstruc- ture-type, via simple annealing treatment of as-deposited Sn on Cu. As will be presented in the following, such electrodes, without any bind- er/conducting additive, resulted in stable cyclic performance and supe- rior Li-capacity retention, which may be comparable to the best (if not the best) achieved to-date with Sn/Sn-intermetallic based electrodes. Furthermore, the simple thin film electrode architecture also allowed monitoring of the in-plane stress developments in-situ during electro- chemical lithiation/delithiation (for the first time with Sn/intermetallic electrodes); throwing some valuable insights into the suppressed me- chanical degradation and significantly improved cyclic stability for such electrodes (developed via simple heat treatment), as compared to the pure (as-deposited) Sn electrodes. Phase pure β-Sn films (see inset of Fig. 1a) were deposited on Cu foil (~30 μm thick), as well as on ~ 100 nm thick Cu-coated quartz wafer (di- ameter ~2.5 cm, thickness ~0.5 mm; for aiding stress measurements), via e-beam evaporation at pressure of 10 -6 mbar using 99.99% pure Sn target (similar to our previously published work [5]). In order to form the Cu-Sn intermetallic phase, the as-deposited films were heat treated at 250 °C for 2 h in Ar atmosphere. Such heat treatment resulted in the formation of Cu 3 Sn and Cu 6 Sn 5 as the intermetallic phases (due to Scripta Materialia 130 (2017) 105–109 ⁎ Corresponding author. E-mail address: amartya_mukhopadhyay@iitb.ac.in (A. Mukhopadhyay). http://dx.doi.org/10.1016/j.scriptamat.2016.11.023 1359-6462/© 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat