Analysis of Germanium Nanoparticle Composites with an Aluminum Current Collector toward Li-ion Battery Anodes with High Capacity and Broad Potential Stability Range

Abstract

Reduction reactions with lithium at >0.3 V vs Li/Li+ make germanium prospectively compatible with aluminum current collectors to form a copper-free lithium-ion cell anode that may enable lithium-ion cells that are more tolerant to overdischarge. Targeted 2.0 mAh/cm2 germanium nanoparticle electrodes with an aluminum foil (Ge–Al electrode) current collector were fabricated and tested versus lithium from 0.3 to 1.5 V vs Li/Li+ in 1.0 M LiPF6 1:1 ethylene carbonate/diethyl carbonate (v/v) electrolyte and demonstrated a peak reversible capacity of 342 mAh/gGe at a targeted C/10 rate. A targeted 2.0 mAh/cm2 germanium nanoparticle electrode with a copper current collector (Ge–Cu electrode) was also tested from 0.005 to 1.5 V vs Li/Li+, the results of which are utilized as a representative comparison of germanium tested in a conventional lithium-ion anode potential range. Compared to the Ge–Cu electrode, after 50 cycles at a targeted C/10 rate the Ge–Al electrode showed 90% vs 18% capacity retention. Post-mortem analysis with XPS, SEM/EDS, Raman spectroscopy, and FTIR showed that compared to the Ge–Cu electrode the Ge–Al electrode had (1) less oxygen, fluorine, and carbon deposition, (2) less germanium amorphization, (3) formation of LixCy and Ge–H, and (4) less Li2O formation. After 150 cycles at equivalent targeted rates, the Ge–Al electrode also had significantly less mid-frequency impedance growth than the Ge–Cu electrode. Modeling of 18650 format LiNiCoAlO2-cathode lithium-ion cells with a Ge–Al anode predicted an achievable energy density and specific energy of up to 491 Wh/L and 198 Wh/kg. A germanium dissolution potential of 4.2 V vs Li/Li+ was assigned based on voltammetry and post-mortem SEM/EDS. Cycling of a Ge–Al electrode with an upper potential cutoff of 4.2 V vs Li/Li+ every tenth cycle showed 30% capacity retention after 50 cycles, an increased capacity fade attributed to solid electrolyte interphase instability up to 4.2 V vs Li/Li+ based on impedance and post-mortem analysis.