RESUMO
Imipramine class tricyclic antidepressants have low ionization efficiencies that make them difficult to detect by using secondary ion mass spectrometry. Ultraviolet picosecond laser desorption postionization (ps-LDPI-MS) is examined here for the detection of four tricyclic antidepressants: imipramine, desipramine, amitriptyline, and clomipramine. About 30 ps laser pulses at either 213 nm (5.8 eV) or 355 nm (3.5 eV) are used for desorption of samples under vacuum, 7.9 eV (157 nm) fluorine laser pulses are used for post-ionization, and the ions so formed are detected by time-of-flight mass spectrometry. Detection of imipramine by 213 nm ps-LDPI-MS shows less fragmentation than either 355 nm ps-LDPI-MS or prior results from 800 nm fs-LDPI-MS. Ionization energies of imipramine, desipramine, amitriptyline, and clomipramine are predicted using density functional theory calculations and used to explain the corresponding ps-LDPI-MS data for these four compounds as resulting from single-photon ionization. The experimental observation of low-mass amine-containing fragments with calculated ionization energies below 7.9 eV is attributed mostly to dissociation during laser desorption, followed by single-photon ionization of the neutral fragments rather than the more traditional mechanism of unimolecular dissociation following single-photon ionization of the parent molecule.
RESUMO
Composite polymer electrolytes (CPEs) are attractive materials for solid-state lithium metal batteries, owing to their high ionic conductivity from ceramic ionic conductors and flexibility from polymer components. As with all lithium metal batteries, however, CPEs face the challenge of dendrite formation and propagation. Not only does this lower the critical current density (CCD) before cell shorting, but the uncontrolled growth of lithium deposits may limit Coulombic efficiency (CE) by creating dead lithium. Here, we present a fundamental study on how the ceramic components of CPEs influence these characteristics. CPE membranes based on poly(ethylene oxide) and lithium bis(trifluoromethanesulfonyl)imide (PEO-LiTFSI) with Li7La3Zr2O12 (LLZO) nanofibers were fabricated with industrially relevant roll-to-roll manufacturing techniques. Galvanostatic cycling with lithium symmetric cells shows that the CCD can be tripled by including 50 wt % LLZO, but half-cell cycling reveals that this comes at the cost of CE. Varying the LLZO loading shows that even a small amount of LLZO drastically lowers the CE, from 88% at 0 wt % LLZO to 77% at just 2 wt % LLZO. Mesoscale modeling reveals that the increase in CCD cannot be explained by an increase in the macroscopic or microscopic stiffness of the electrolyte; only the microstructure of the LLZO nanofibers in the PEO-LiTFSI matrix slows dendrite growth by presenting physical barriers that the dendrites must push or grow around. This tortuous lithium growth mechanism around the LLZO is corroborated with mass spectrometry imaging. This work highlights important elements to consider in the design of CPEs for high-efficiency lithium metal batteries.
