Fracture of crystalline germanium during electrochemical lithium insertion

Germanium is one of the promising alloying anode (Si, Ge, Sn) materials for high capacity lithium ion batteries. Since it is isostructural with crystalline silicon, crystalline Ge is expected to show intriguing lithiation-induced phenomena similar to Si, such as anomalous volume expansion and fracture. Here, we present the study of lithiation of Ge micropillars, and we compare the findings to silicon pillar lithiation. The critical pillar diameter ∼1.2μm associated with lithiation-induced fracture of 〈111〉 Ge pillars, determined through our statistical investigation, is much greater than the critical dimension for fracture of 〈111〉 silicon pillars (∼300 nm). This larger critical size for lithiation-induced fracture of Ge likely arises from lower tensile stress concentrations at the surface due to the more inherently isotropic expansion that Ge undergoes during lithiation. Upon lithiation, Ge displays only slight anisotropy in its volume expansion, with the 〈110〉 directions exhibiting radial expansion that is only 1.1 times larger than that along 〈111〉 directions. Despite its relatively weak anisotropy in volume expansion, however, Ge pillars above the critical dimension still show anisotropic fracture, with favored fracture sites residing between the 〈110〉 directions on the pillar sidewall, similar to Si. We believe that this study provides better understanding of lithiation of Ge for designing high-density anode for Li-ion batteries.