Germanium (Ge) is a promising candidate for high-capacity Li-ion battery anode due to its high theoretical capacity of 1384 mAh/g (for the charged Li₁₅Ge₄ phase), low operation voltage, fast bulk Li diffusion, and high electrical conductivity. However, the major challenge in the development of Ge anode is the large volume change involved in the reaction scheme, which could result in rapid loss of specific capacity. We recently reported that selenium (Se) -doped Ge (GeSe) forms an inactive phase that buffers the volumetric expansion of Ge which provides better cycling life and performance (1). However, the dynamic morphological and chemical transformations of GeSe that occur during battery cycling processes is still elusive.

The traditional in-situ coin cell has rotating limitations which makes three-dimensional experiment difficult except via limited angle tomography. It also suffers from X-ray damage problems to the binder. To understand the dynamic morphological and chemical changes of GeSe, we developed a single particle battery cell for operando TXM and XANES. A micron-sized GeSe particle was attached to a pre-coated tungsten probe by ion beam carbon deposition on a Zeiss Nvision 40 FIB-SEM at the Center for Nanoscale Materials, Argonne National Laboratory. The single particle battery cell was assembled in an argon-filled glovebox. A small piece of Li metal was attached to a copper wire as the counter electrode. Both the tungsten and lithium electrodes were carefully inserted into a funnel-shaped open-ended silica capillary and stabilized via epoxy. The quartz housing was fully filled with 1 M LiPF₆ EC/DEC electrolyte through a micro-syringe and sealed with torr seal epoxy. This binder free operando single particle cell eliminates discrepancies caused by particles overlapping in the in-situ coin cell during TXM and the impact of X-ray damage to the binder (2). It provides chemical and morphological information in three-dimensional space via computed nanotomography coupled with XANES. Figure 1 shows the performance of operando GeSe cell and the 2D morphological change of a GeSe particle attached to the tungsten probe at 11.113 keV. Regardless of the defect caused by the carbon deposition process, the results indicate that GeSe undergoes a different dynamic chemical transformation compared to pure Ge. The selenium-doped germanium has the ability to reinstate its original shape after cycling. This study can help to understand the mechanical stability and degradation mechanism of high capacity lithium alloy anode with additional stress-accommodating phase.

1. K. C. Klavetter, J. Pedro de Souza, A. Heller, C. B. Mullins, High tap density microparticles of selenium-doped germanium as a high efficiency, stable cycling lithium-ion battery anode material. Journal of Materials Chemistry A 3, 5829-5834 (2015).
2. L. Y. Lim, N. Liu, Y. Cui, M. F. Toney, Understanding Phase Transformation in Crystalline Ge Anodes for Li-Ion Batteries. Chemistry of Materials 26, 3739-3746 (2014).