Due to their high specific capacities and higher (safer) operating potentials, metallic materials such as Sn and Al are among the promising alternatives to graphitic carbon as anode materials for Li-ion batteries. However, the metallic anode materials undergo huge volumetric changes during lithiation/delithiation which leads to the development of stresses that result in severe mechanical degradation and concomitant drastic capacity fade. Furthermore, the stresses affect the thermodynamics of the lithiation/delithiation ‘reactions’ and exert influences on the various electrochemical behaviour. In order to develop better understanding of the origins, magnitudes and resulting effects of such stress developments, in correlation with the initial stress states, crystallographic orientations, stages of lithiation/delithiation, associated phase transformations and surface reactions, we have monitored the stress developments in-situ during lithiation/delithiation. Sn and Al have been selected for the present studies, with the concerned electrodes possessing simple thin film architectures, without binder/additive/porosity.

The active electrode films of ~100 nm in thickness have been deposited via e-beam deposition on different current collector films (~100 nm thick), such as Cu (for both Sn and Al), Ni (for Sn) or Ti (for Sn) on thicker (~0.5 mm) quartz substrates (stiff substrate) for allowing monitoring of the in-plane stress developments via substrate curvature technique using multi-beam optical stress sensor (MOSS) in-situ during electrochemical cycling against Li metal in custom-made electrochemical cell [1]. In addition to the expected developments of compressive stresses during lithiation and reverse during delithiation, we focus on some of the finer features of the stress developments, which involves the stress evolution during the first order phase transformations that take place during the Li-alloying reactions (i.e. formation of the various Li-Sn and Li-Al intermetallic) [2]. To develop better understanding, attempt has also been made to mathematically model the observed in-plane stress developments at the different stages (single and two-phase regimes). As will be demonstrated, our observations suggest occurrences of mechanical degradation mainly during the phase transformations, with the electrode materials being fairly stable during the single phase (solid solution) regimes.

In order to explore a possible route towards improving the mechanical integrity and cyclic stability upon lithiation/delithiation, the effects of inducing pre-cycling residual stresses via simple annealing treatments have been investigated. Carefully planned experiments with films possessing different residual stress states, aided with in-situ monitoring of the stress developments during lithiation/delithiation, have highlighted the importance of such initial conditions on aspects related to cyclic stability and overpotentials. The use of Sn and Al based electrodes, where the molar volume change associated with the phase transformations are considerably higher compared to other electrode materials, have further strengthened the understanding. On a different note, the influences of different current collectors (Cu, Ni and Ti) on the crystallographic orientations, and concomitantly on the various electrochemical behaviour and stress developments will also be highlighted. Our in-situ observations also suggest the development of stresses due to SEI formation, similar to the case of graphitic carbon [3]. It is believed that the understandings developed as part of the present work will contribute towards successful developments of electrode materials based on alloying reactions.

Keywords: anode materials, Sn, Al, stress development

References

1. A. Mukhopadhyay, B. W. Sheldon; Prog. Mater.Sci. 63 (2014) 58

2. A. Mukhopadhyay, R. Kali, S. Badjate, A.Tokranov, B.W. Sheldon; Scripta Mater. 92 (2014) 47

3. A. Mukhopadhyay, A. Tokranov, X. Xiao, B. W. Sheldon; Electrochim. Acta 66 (2012) 28

Acknowledgements
The authors acknowledge DST and CSIR, Government of India, and IRCC, IIT Bombay for financial supports and SAIF, IIT Bombay for allowing the usages of different characterization techniques.