Nanoscale Operando and 3D In Situ Imaging of Micron-Sized Ge Anode Particles in Li Batteries

Ge anodes have very high theoretical lithium capacity (1600 mAhg^-1) and are promising anodes for Li batteries. Ge is more electronically conductive than Si and has a higher room temperature Li-ion diffusivity. However, because of their high capacity, Ge anodes suffer from large volume changes (up to 400%; similar to Si, at 4200 mAhg^-1) during electrochemical cycling. Repeated expansion and contraction of these materials lead to fracturing and eventual pulverization resulting in the loss of electrical contact with the current collector and ultimately battery failure.

In order to better understand these failures we have conducted the first 3D nanoscale in situ study of a Li battery system. We imaged Ge anodes with hard X-rays (11.2 keV, above the Ge K-edge) with transmission X-ray microscopy at 30 nm resolution. 2D images in operando were collected at several regions within the particles, and 3D tomography data was also collected at different time points within the fully operating battery (in situ) to further examine the crack formation and density changes that occur with cycling. We found that volume expansion during lithiation is particle size dependent, but the subsequent volume contraction is not. Moreover, we found that only the largest particles actively participate in the second (de)lithiation cycle. From the in situ tomographic information we quantified the volume change in particles and measured the changes in porosity or density as particles (de)lithiate. Our results demonstrate the significant value of high resolution 3D imaging of batteries under operating conditions.