Single-crystalline Si represents not only the material that allows the realization of monolithic integrated circuits, it also possesses outstanding lithium uptake capabilities. In contrast to the most common anode materials for which lithiation proceeds according to topotactic insertion as well as transformation mechanisms, the reaction of lithium with silicon is different: a “ledge-mechanism” is present which means Si layer after Si-layer is peeled off [1]. During lithiation the silicon which is detached transforms to an amorphous Li_xSi-phase [2, 3] with fast Li-ion diffusivity [4]; the Si-layers located far from the surface remain untouched and, thus, mechanically resilient.

Under these conditions and because of the lithiation mechanism, highly doped single-crystalline silicon may perform two specific functions in a battery integrated on a chip: (i) as both current collector and housing material and (ii) as powerful anode material with a maximum charge density of 3579 mAhg^-1, which is electrochemically accessible [2].

In the present work a novel µ-battery with an anode and a case of highly doped semiconductor-grade single crystalline silicon, a liquid electrolyte and an NCA-cathode is introduced. The battery can be placed on the free silicon backside of a microchip. While SEM/FIB-measurements were used to characterize the morphological changes of the material, the electrochemical performance of the system was thoroughly studied. The battery as it is now shows excellent cycling performance: it can be cycled for more than hundred times at specific anode charge densities as high as 1000 mAhg^-1and coulombic efficiencies better than 99,8 %.

Acknowledgment

Financial support by the Federal Ministry of Economy, Family and Youth and the National Foundation for Research, Technology and Development is gratefully acknowledged. Moreover, we thank the Institute for Electron Microscopy and Nanoanalysis, Graz University of Technology and Graz Centre for Electron Microscopy.

References

[1] Liu, X. H.; Wang, J. W.; Huang, S.; Fan, F.; Huang, X.; Liu, Y.; Krylyuk, S.; Yoo, J.; Dayeh, S. A.; Davydov, A. V.; Mao, S. X.; Picraux, S. T.; Zhang, S.; Li, J.; Zhu, T.; Huang, J. Y., In situ atomic-scale imaging of electrochemical lithiation in silicon. Nat Nano 2012,7, (11), 749-756.

[2] Obrovac, M. N.; Christensen, L., Structural Changes in Silicon Anodes during Lithium Insertion/Extraction. Electrochemical and Solid-State Letters 2004,7, (5), A93-A96.

[3] Hatchard, T. D.; Dahn, J. R., In Situ XRD and Electrochemical Study of the Reaction of Lithium with Amorphous Silicon. Journal of the Electrochemical Society 2004,151, (6), A838-A842.

[4] Dunst, A.; Sternad, M.; Epp, V.; Wilkening, M., Fast Li+ Self-Diffusion in Amorphous Li–Si Electrochemically Prepared from Semiconductor Grade, Monocrystalline Silicon: Insights from Spin-Locking Nuclear Magnetic Relaxometry. The Journal of Physical Chemistry C 2015,119, (22), 12183-12192.

Figure 1: Outer casing of the µ-battery (left) and SEM image of a Focused-Ion Beam (FIB) cut through the tower structures of a silicon anode that was charged and discharged several times (right).