Evolution of Nano-Crystalline Silicon Anode upon Cycling Probed By E x Situ Raman Spectroscopy and Operando Synchrotron X-Ray Diffraction

Silicon is being considered as one of the most promising anode materials for next generation lithium-ion batteries due to its high theoretical capacity (3580 mA h g -1 at room temperature) (1). Upon charging, Si and Li react via an alloying process with a sharp interphase separating the growing Li x Si amorphous phase from the pristine crystalline Si. In case of spherical particles this leads to a core-shell structure captured in Figure1a. The alloying process leads to an enormous volume expansion of Si resulting in cracking of the electrode material and rapid fading of its performance that impedes the commercialization of Si-based anodes. Nevertheless below a critical particle diameter of 150nm volume expansion does not lead to cracking (2). In order to profit from silicon unique properties, the structural evolution along charging/discharging must be understood. Using ex situ Raman spectroscopy we observed the gradual transformation of initially crystalline silicon into amorphous one upon cycling. The resulting stresses generated in the crystalline cores of the core-shell structures were probed as well as their evolution along lithiation/delithiation. The changes of the crystalline lattice during the initial cycles were also monitored using operando synchrotron X-ray diffraction (XRD). In addition we were able to further quantify the constrained/unconstrained state of the silicon core. We show that combined ex situ Raman spectroscopy and in situ XRD provide in-depth understanding of the nanoparticles evolution. Figure 1 . Schematic representation of silicon electrodes: a) Formation of a core-shell structure upon cycling, b) Influence of the laser power on the Raman signal and c) Gradual amorphization of the electrode along cycling (All spectra were normalized to the maximum of the c-Si peak) 1. Obrovac, M. N., and Leif Christensen. "Structural changes in silicon anodes during lithium insertion/extraction." Electrochemical and Solid-State Letters 7.5 (2004): A93-A96. 2. Liu, Xiao Hua, et al. "Size-dependent fracture of silicon nanoparticles during lithiation." Acs Nano 6.2 (2012): 1522-1531. Figure 1