Reversible interfacial Li-oxide formation on germanium and silicon anodes revealed by time-resolved microgravimetry

Abstract

Germanium has received recent attention as a promising candidate for the next generation of Li-ion anodes due to its high capacity, higher conductivity, and faster lithium-ion diffusivity relative to silicon. However, the uncertainty in the Li-storage mechanism and the formation of a solid electrolyte interface (SEI) has hampered its further development. Herein, operando microgravimetry on the Ge electrode has been conducted to determine the SEI mass (irreversibly deposited) and to distinguish it from the reversibly cycled species, and their individual contributions to the total capacity. From the obtained time-resolved mass spectra, determined by correlating gravimetry and charge flux, Li₂O along with Li is identified as a reversibly cycled species, contributing significantly to the electrochemical capacity. The amount of this cycled Li₂O depends on (i) the cycling rate and (ii) the thickness of the anode. Interestingly, the amount of Li₂O decreases if pronounced anode cracking appears. A direct comparison between Si and Ge indicates that the amount of the reversible Li₂O is predominantly controlled by the diffusivity of lithium into the electrodes. The study provides a general mechanism for balancing the interfacial and bulk storage of Li in the form of an oxide or by alloying, respectively.