b S Supporting Information L ithium ion batteries (LIBs) hold great promise for demand-ing applications such as electric vehicles. 1,2 Silicon is being considered as a candidate anode material in LIBs, as it possesses the highest specific capacity among all materials (∼3579 mAh g À1 for Li 15 Si 4), 3À6 over an order of magnitude higher than that of carbonaceous anodes used in current LIBs. However, during electrochemical lithiation, Si electrodes experience large volume expansion up to ∼300%, leading to pulverization and huge capacity loss even in the first cycle. 1,7À25 While Si has been intensively studied in real and bulk electrochemical cells, 4,5,7À12,15,19,26À32 and impressive colossal volume change was directly observed with in situ optical microscopy and atomic force microscopy, 7,26 it still remains unclear how cracks initiate and evolve at the atomic scale. Although decrease of Si to nanoscale can alleviate pulverization, and nano-Si has been recognized as a promising anode for Li-ion batteries, the poor cyclability still remains and the mechanism behind is not fully understood. 9,10 A fundamental understanding of the deformation and fracture mechanisms in lithiated Si may help develop strategies to accommodate or mitigate the large expansion and crack formation during cycling, thus paving the way for the application of Si as a high energy ABSTRACT: We report direct observation of an unexpected anisotropic swelling of Si nanowires during lithiation against either a solid electrolyte with a lithium counter-electrode or a liquid electrolyte with a LiCoO 2 counter-electrode. Such aniso-tropic expansion is attributed to the interfacial processes of accommodating large volumetric strains at the lithiation reac-tion front that depend sensitively on the crystallographic orientation. This anisotropic swelling results in lithiated Si nanowires with a remarkable dumbbell-shaped cross section, which develops due to plastic flow and an ensuing necking instability that is induced by the tensile hoop stress buildup in the lithiated shell. The plasticity-driven morphological instabilities often lead to fracture in lithiated nanowires, now captured in video. These results provide important insight into the battery degradation mechanisms.