Silicon and its Challenging Interfaces
Authors: Edström Kristina , Ma Yue , Philippe Bertrand , Xu Chao , Lindgren Fredrik , Rensmo Hakan , Gustafsson Torbjörn 
Published: February 2020

The optimal silicon electrode that should have a practical capacity of roughly 1200 mAh/g for a balanced lithium-ion cell is a dream and thus a challenge. Pulverization of silicon particles and loss of electrical contact have been identified as the main causes for the performance deterioration. Binder formulations, cut-off window for the cycling, and electrolyte composition with different additives are some routes to take to improve the long-term cycling stability of the electrode. In this presentation, we will especially focus on characterization of the SEI and the buried interface below the SEI which can give us new light on how to improve the use of silicon 1,2,3,4 We will discuss how different lithium salts can give very different SEI compositions and buried interfaces. We will also show how some of the salts will give SEIs that are porous and other that lead to a continuous growing SEI as a function of cycling. The role of state of charge on the growth of SEI will be discussed.
The lessons we have learnt about the SEI on silicon will be dwelt upon by giving examples on strategies to improve cycling stability. We will show how a 3D hierarchical arrangement of spatially confined Si nanocrystals, within several different physical insulting capsules, can be one way of improving the performance. Firstly, an elastic, highly oriented graphene monolith (GF) has been engineered to fully encapsulate Si nanoparticles, serving as a robust framework with accessible thoroughfares for electrolyte percolation. The framework is also acting as an electrolyte blocking layer to restrain Si from direct exposure to electrolyte without sacrificing the highly efficient electron/Li ion transport channels. The Si nanoparticles are arranged in pillars within the hierarchical structure. This seems to be to prevent the graphene sheets from re-stacking. Secondly, a TiOxFy layer was grown on the silicon surface to support in-situ etching of the native oxide layer into a hollow interior. When evaluated as binder-free anodes, both types of electrodes exhibit cycle life for more than 1000 cycles with average coulombic efficiencies higher than 99.5%. Operando XRD and synchrotron-based XPS confirmed the formation of the most volume expanded lithiated phase Li15Si4, suggesting the effective buffering of the volume variation upon cycling. Furthermore, we have also developed a GF/Si free-standing film through re-adjusting the pore size in GF/Si monolith. Thanks to high electrical conductivity, the elasticity and structural integrity of GF monoliths, the as-developed GF/Si free-standing film showcases the potential use in the flexible electronic devices.
The engineering of interfaces in relation to optimizing electrolyte composition are important strategies for the future of stable LIBs.

References

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Acknowledgments

We acknowledge HZB for the allocation of synchrotron radiation beamtime. The research leading to these results has received funding from the European Community's Seventh Framework Program (FP7/2007-2013) under grant agreement n.°312284. The authors are also grateful to StandUp for Energy and the Swedish Research Council (contract 2012-4681) for financial support.