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radiation heats the silicon beyond the melting point. ... irradiation with laser pulse energy density ... Advanced electrolytes, with fluoroethylene carbonate as.
Porous Silicon Thin Film Anodes for Lithium Ion Batteries Christian Saemann1, Katerina Kelesiadou1, Seyedeh Sheida Hosseinioun2, Mario Wachtler2, Juergen Koehler1, Peter Birke1, Markus Schubert1, and Juergen H. Werner1 1 2

Institute for Photovoltaics and Research Center SCoPE, University of Stuttgart, Germany ZSW – Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg, Ulm, Germany Corresponding e-mail: [email protected]

Secondary batteries offer the opportunity to store the increasing amount of surplus electricity from fluctuant renewable energies like solar or wind energy. When solar or wind power cannot provide enough electricity to the grids, electrochemically stored energy closes the gap of demanded electricity. Therefore, an increase of the specific capacity of batteries is necessary in order to keep up with the growth of installed renewable energy power. For electric vehicles, an improvement of the specific energy of their batteries results in an extended range of the vehicle with equal weight, which is important in the competition with internal combustion engine vehicles. Thus, research on new materials for batteries is required in order to increase their specific capacity for portable as well as for stationary applications. One promising approach lies in replacing the active material graphite of the negative electrode (anode) in conventional lithium ion batteries with silicon. Silicon has the highest known specific capacity CSi = 3579 mAh/g (Li15Si4) [1] for lithium, thus has a theoretical potential of increasing the specific capacity by a factor of 9 in comparison to the specific capacity of graphite with CC = 371 mAh/g [2]. Silicon, unfortunately, expands during lithiation up to 270% in volume. This volume expansion causes mechanical damage of silicon during lithiation and delithiation especially in the first cycle by cracking or fracturing and often results in the loss of electrical contact to the current collector. Figure 1 illustrates the mechanical damage of silicon induced by volume expansion. During delithiation, stress in the silicon film causes cracking and loss of electrical conduction to the current collector. Rapid capacity fading is the consequence, finally leading to a total failure of the silicon anode.

Figure 1: Due to volume expansion of up to 270%, silicon as active material on metal foil current collectors fractures during electrochemical cycling. The cracks in between the silicon islands and the loss of electrical contact between the metal foil and single silicon islands result in capacity fading and total failure. To prevent capacity fading during lithiation/delithiation, therefore, a change of the morphology of silicon is required in order to increase the stability for such volume changes. Porous silicon, for example, promises to fulfill these requirements. The pores improve the non-destructive volume change capacity by allocating space for the expansion. Quite naturally, they reduce the induced stress, thus leading to a more stable capacity retention. However, simple methods are necessary to produce such pores.

This contribution presents a novel method to produce porous thin film silicon for negative electrodes in thin film lithium ion batteries by pulsed laser radiation [3]. We deposit, either by sputtering with argon or by plasma enhanced chemical vapor deposition (PECVD), several 100 nm thick amorphous silicon layers on metal foil current collectors. Sputtering results in an incorporation of argon in the sputtered films [4]. PECVD uses silane (SiH4) as process gas, which results in incorporation of hydrogen from the process gas molecule. A pulsed laser beam of wavelength λ = 532 nm in shape of a line locally irradiates the silicon thin films. The pulsed laser beam is scanned over the surface of the film. Strong absorption of the green laser radiation heats the silicon beyond the melting point. As soon as silicon undergoes the phase change from solid to liquid, incorporated gas atoms merge to gas bubbles. Further increase of the temperature during the laser pulse induces thermal expansion of the gas bubbles. Finally, they reach the surface and leave the film. After the laser pulse of 100 ns ≤ τP ≤ 250 ns duration, a fast cooling and solidification of silicon creates the porous structured and laser crystallized thin film silicon. Figure 2 shows a scanning electron microscopy (SEM) image of Ar-sputtered dSi = 200 nm thick silicon on a stainless steel foil substrate after irradiation with laser pulse energy density HP = 0.64 Jcm-2 and pulse duration τP = 220 ns. The pores have diameters up to 300 nm and improve the stability for lithiation. We are able to tailor the diameter distribution and density of pores by varying the laser pulse energy density HP and the pulse duration τP. The electrochemical performance of the porous silicon materials is tested in lithium half-cells using capacity-limited galvanostatic cycling. The performance of the silicon negative Figure 2: SEM image of porous silicon on stainless electrodes depends on the porosity created by steel foil substrate after treatment by laser pulse the used laser parameters. The porous thin film energy density HP = 0.64 Jcm-2 and pulse duration silicon negative electrodes show an improved τP = 220 ns. The created pores stabilize silicon under cyclic stability compared to silicon anodes without any laser treatment. Thus, the porosity volume expansion during lithiation/delithiation. of silicon increases the mechanical stability for the high volume changes. Cycling with a conventional electrolyte shows a stable performance of the porous silicon for more than 100 cycles. Advanced electrolytes, with fluoroethylene carbonate as additive, yield a stable cycling for more than 500 cycles, because fluoroethylene carbonate stabilizes the solid electrolyte interphase [5]. The coulombic efficiency ηc of the silicon anodes reaches over 98% after the initial solid electrolyte interphase formation. Therefore, laser porosification of thin film silicon in combination with advanced electrolytes is promising to adjust the morphology and the solid electrolyte interphase of silicon. Such silicon films might replace graphite as an active anode material in lithium ion batteries. [1] M. N. Obrovac and L. Christensen, Electrochem. Solid-State Lett., 7, A93 (2004) [2] R. Fong, U. Von Sacken, and J. R. Dahn, J. Electrochem. Soc., 137, 2009 (1990) [3] C. Sämann, J. R. Köhler, M. Dahlinger, M. B. Schubert, and J. H. Werner, Materials, 9, 509 (2016) [4] H. F. Winters and E. Kay, J. Appl. Phys., 38, 3928 (1967) [5] N.-S. Choi, K. H. Yew, K. Y. Lee, M. Sung, H. Kim, and S.-S. Kim, J. Power Sources, 161, 1254 (2006)