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Using an ultrastable continuous-wave laser at 580 nm we performed spectral hole burning of Eu(3+):Y(2)SiO(5) at a very high spectral resolution. The essential parameters determining the usefulness as a macroscopic frequency reference, linewidth, temperature sensitivity, and long-term stability, were characterized using a H-maser stabilized frequency comb. Spectral holes with a linewidth as low as 6 kHz were observed and the upper limit of the drift of the hole frequency was determined to be 5±3 mHz/s. We discuss the necessary requirements for achieving ultrahigh stability in laser frequency stabilization to these spectral holes.

To improve the lifetime of lithium-ion batteries, a detailed understanding of the degradation mechanisms is essential. Nuclear magnetic resonance (NMR) is able to unravel the reversible as well as irreversible transient changes of composition, shape and morphology in a battery cell. Using a newly developed cylindrical battery container free of metallic components in combination with a numerically optimized saddle coil, in operando NMR investigations of battery cells over hundreds of charge/discharge cycles are presented. Alternating with NMR data acquisition, electrochemical impedance spectra (EIS) can be recorded, which enables correlative analysis of the two techniques. Long-run in operando NMR measurements on a Li metal vs. graphite cell reveal the formation and evolution of mossy and dendritic Li microstructures over a period of 1000 h, which illustrates the capabilities of NMR to identify dendrite mitigation strategies in cells operated under realistic conditions.