α-Ag2S nanoparticles: low-temperature syntheses, crystallisation pathway, and first operando luminescence measurements for sodium-ion battery applications

Ag2S nanoparticles (NPs) are attractive materials for wide application in catalysis, solar cells, and energy storage, owing to their low toxicity, tunable electronic properties, narrow bandgap, and light emission. However, in situ and operando studies are required for a deeper understanding of the crystallisation pathways that govern their structural-related properties and changes during operation, for example, during galvanostatic discharge–charge (GDC) in sodium-ion batteries (SIBs), to improve their cycling performance. This work presents new strategies for the synthesis of α-Ag2S NPs applying aqueous or polyol-assisted methods, and using double- or single-source based approaches. Double-source polyol synthesis was used to demonstrate the effect of reaction time, capping agent, and reaction temperature on the Ag2S particle size. Insights into Ag2S formation processes were obtained by in situ analysis of changes in turbidity and of infrared or UV/vis transmittance spectroscopy, in addition to synchrotron-based X-ray diffraction. After testing the application of Ag2S NPs as SIB electrodes, a proof-of-principle experiment is presented, demonstrating the feasibility of measuring operando luminescence spectra to monitor changes in the electrode structure during GDC cycling. The in situ results showed the influence of synthesis parameters on the nucleation process, resulting in compact and hollow NPs of ∼5–400 nm. Ag2S NPs showed an initial capacity of 520 mA h g−1 in SIBs. Despite an expected large capacity loss due to e.g. the solid electrolyte interphase (SEI) formation, the capacity was found to be stable, and operando luminescence measurements indicated the formation of Na(Ag3S2) and elemental Ag during cycling. Since several battery-electrode materials are optically active, operando luminescence measurements during cycling significantly impact the monitoring of structural changes owing to their high availability and independence from synchrotron radiation.