Silicon has attracted substantial attention as an alternative anode material for next-generation Li-ion batteries (LiBs) as it has a high theoretical capacity of 3580 mA h g-1 that is about ten times that of the commonly used graphite. (1) Upon charging, Si and Li react via an alloying process with a sharp interphase separating the growing LixSi amorphous phase from the pristine crystalline Si. In case of spherical particles this leads to a core-shell structure captured in Figure1b and c. An enormous volume expansion of Si associated with alloying leads to cracking of the electrode material and rapid fading of its performance. This hinders Si from being widely used in LiBs. It has been reported that for Si nano-particles (diameter<150nm) volume expansion does not lead to cracking (2). To benefit from silicon unique properties and enable realization of smart electrode design, an enhanced understanding of structural evolution along charging/discharging is required.
In the present work we were able to evidence the gradual amorphisation of crystalline Si nanoparticles upon cycling using ex situ Raman spectroscopy. Furthermore we probed the resulting stresses found in cores of the core-shell structures of Si nanoparticles and their evolution along lithiation/delithiation. Operando synchrotron X-ray diffraction (XRD) has also been performed to monitor the changes of the crystalline lattice during the initial cycles and to further quantify the constrained/unconstrained state of the silicon core. By means of combined ex situ Raman spectroscopy and in situ XRD we were able to provide in-depth understanding of the nanoparticles evolution.
Figure 1. Schematic representation of silicon electrodes: a) pristine b) after 1 cycle lithiation c) after 100 cycles lithiation d) corresponding Raman spectra. Black, blue and green, respectively. (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.