Fracture of germanium pillar during electrochemical lithiation
Seok Woo Lee (Stanford University), Ill Ryu (Stanford University), Lucas Berla (Stanford University), Matthew McDowell (Stanford University), William Nix (Stanford University), Yi Cui (Stanford University)
Lithium ion batteries: When Chemistry meets Mechanics
Mon 9:00 - 10:30
Salomon 003
Alloying anode materials are considered promising anode electrode for Li-ion batteries because of its exceptional specific capacity due to severe structural transformation. However, conventional alloying anodes typically suffer from rapid capacity decay due to mechanical fracture caused by large volume changes during repeated electrochemical lithium insertion and extraction. Hence, various fundamental studies of how anode materials expands and fracture occurs during lithiation and delithiation have been done. Especially, silicon has been extensively studied due to highest specific capacity and abundance among alloying materials. Our previous studies presented anomalous volume expansion behavior of crystalline Si nanostructures during lithiation that contradict the commonly held belief that volume expansion of Si occurs via isotropic Li reaction and diffusion. First, anisotropic volume expansion and anomalous fracture behavior of crystalline Si nanostructures of various axial orientations has been shown. Here, we present the study of lithiation of germanium pillars comparing with silicon pillars. Germanium is expected to show similar lithiation behavior as silicon does since it is also IV semiconductor material with diamond cubic crystalline structure. This study reveals that germanium has less anisotropic expansion behavior, which is only 1.1 times larger expansion along < 110 > direction than along < 111 > direction. But, favored fracture site between each < 110 > directions of the germanium pillar is still found in spite of less anisotropy. The statistic study of size effect of fracture of < 111 > germanium pillars shows much larger critical fracture diameter than of silicon pillars. Larger critical size of fracture of germanium might be caused by less tensile stress concentration at the surface due to less anisotropic expansion during lithiation.