Insights into the Li-Storage Mechanisms and Mechanical Integrities of the Next Generation Anode Materials for Li-Ion Batteries

The issues concerning safety and relatively inferior Li-capacities of graphitic carbon based anodes for Li-ion batteries have motivated researchers world-wide to look at alternative anode materials such as metals, graphenic carbon and also metal/carbon composites. Despite extensive research efforts, some of the major issues and behavioral patterns associated with these anode materials are either yet to be resolved, or better understood in scientific terms. For instance, the metallic anode materials still suffer from drastic capacity fades due to mechanical degradation arising from huge volumetric changes upon lithiation/delithiation [1]. Furthermore, still lacking is the understanding of the correlations between the potential, state-of-charge, phase transformations and stress developments in such electrode materials. With respect to graphenic carbon based anodes, even though recent reports suggest significantly enhanced Li-capacities, the Li-storage mechanisms leading to such capacities are still under debate [2,3]. Furthermore, the mechanical integrity of graphenic carbon upon repeated lithiation/delithiation has not yet been established. Here it is interesting to note that, even though as per the common belief graphene may be used to buffer the stresses in the metallic anode materials [4], this possible role of graphene is yet to be understood in scientific terms, especially in terms of quantifying the stress developments in the presence or absence of graphene.

Against these backdrops, in order to develop better fundamental understandings on these issues we are using thin films of Sn, Si, few layer graphene (FLG) and Si-FLG multi-layered films as model electrode materials/architectures. The Sn and Si films are deposited via e-beam deposition, while the FLG (~ 6 layers) films are deposited via chemical vapor deposition on Cu foil/film (using methane; at 1000^oC). In addition to studying the electrochemical behaviour using different current densities and potential/current steps against Li-foil, real-time monitoring of the in-plane stress developments have been performed using multi-beam optical stress sensor during electrochemical cycling [1,5]. Such in-situ investigations have allowed quantitative determinations of the stress developments and provided us with deeper insights into the various unresolved aspects; for eventually establishing correlations between the potentials, surface phenomena, state-of-charges, Li-storage mechanisms, phase/stage transformations, dimensional changes and stress developments.

With respect to lithiation/delithiation of Sn, we have observed for the first time that the stress developments remain elastic in the single phase (solid solution) regimes, whereas plastic deformation and/or fracture occur just during the first order phase transformations between the different Sn-Li intermetallics during Li-alloying/dealloying [5]. It has been hypothesized that the huge stresses associated with the eigenstrains related to the formation of the new intermetallics with significantly different molar volumes than the parent phases are responsible for this behaviour. Our work with few layer graphene based thin films has allowed us to develop better insights into the respective contributions from the classical Li-intercalation mechanism and Li-storage in defect sites towards the overall Li-capacity, which we re-confirm as being significantly higher compared to that for bulk graphitic carbon. Additionally, our in-situstress measurements reveal that mechanical degradation do occur in each electrochemical cycle during the initial stages of lithiation and later stages of delithiation; more specifically, during the co-existences of dilute stage I and stage IV Li-GICs. As will be presented, our analysis correlates such structural degradation with the Li-distribution near the edge sites and differential inter-planar spacing in FLG during these stages. Our work with graphene (FLG)/Si multi-layered films has revealed that just the presence of graphene may not be successful in improving the structural integrity of the composite anode for enhancing cycle life. The behavioral patterns and overall stress magnitudes obtained in real-time during electrochemical cycling allow better understanding on the influence of the graphene layer and highlights the need for careful optimization of the relative thicknesses of the Si and graphene films and the lateral dimensions of the Si islands (in case of patterned Si films) for improving the effectiveness of graphene as a ‘buffer layer’.

Acknowledgements: The contributions from Ravi Kali, Farjana J. Sonia, Manoj K. Jangid, Dr. M. Aslam (IIT Bombay, India); and Anton Tokranov and Prof. Brian W. Sheldon (Brown University, USA) are duly acknowledged. Financial assistances from IRCC, IIT Bombay and DST & CSIR, Government of India are duly acknowledged.

References

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3. E. Lee, K.A. Persson; Nano. Lett. 12 (2012) 4624.
4. V. Chabot, K.  Feng, H.W.  Park, F.M.  Hassana, A.R. Elsayed, A.  Yu, X.  Xiao, Z.  Chen; Electrochim. Acta 130 (2014) 127.
5. A. Mukhopadhyay, R. Kali, S. Badjate, A. Tokranov, B.W. Sheldon; Scripta. Mater. 92 (2014) 47.