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Silicon has attracted significant attention for use as the negative electrode material in Li-ion batteries [1–5] in part because it has a specific capacity about ten times that of commercial graphite. In contrast to traditional intercalation electrodes, Si and Li react via an alloying process, which results in an enormous (∼ 300%) volume expansion due to the uptake of up to 4.4 Li atoms per Si atom in the fully alloyed material.[6, 7] This large volume change is a fundamental issue that has hindered the widespread use of Si in batteries. Concentration gradients associated with volume expansion can cause significant mechanical stress to exist within electrode structures, which can lead to fracture and electrical isolation of the active material.[8–11] In addition, volume expansion/contraction of Si structures can promote unstable growth of the solid-electrolyte-interphase (SEI), which results in high ionic resistance and poor cycling performance.[12] Fracture compounds this issue: the SEI grows on the new surfaces of a fractured electrode particle, which could promote electrical isolation since the SEI is electronically insulating.[13] In short, the volume changes during Li alloying/dealloying are the cause of many problems associated with this system, so it is vital to understand the fundamental nature of these volume changes and how they relate to the reaction itself. The electrochemical properties of the Li-Si system have been studied for more than three decades,[14, 15] but research on the effects of volume expansion has only been performed more recently.[7, 16] In the past few years, a number of studies have specifically focused on lithiation and volume …