RESUMEN
The realization of large powerful all-solid-state batteries is still hampered by the availability of environmentally friendly and low-cost Li ion conductors that can easily be produced on a large scale and with high reproducibility. Advanced solid electrolytes benefit from fast ion-selective transport and non-flammability, but they may have low electrochemical stability with respect to Li metal. Sol-gel-synthesized lithium titanium aluminum phosphate Li(1.5)Al(0.5)Ti(1.5)(PO4)3 (LATP), which was prepared via a new synthesis route taking advantage of an annealing step at relatively low temperatures, has the potential to become one of the major players in this field although it may suffer from reduction upon direct contact with metallic lithium. Its ion dynamics is, however, as yet poorly understood. In the present study, (7)Li nuclear magnetic resonance (NMR) spectroscopy was used to monitor the key Li jump processes on the atomic scale. NMR relaxation clearly reveals heterogeneous dynamics comprising distinct ultra-fast and slower diffusion processes. The high Li ion self-diffusion coefficients deduced originate from a rapid Li exchange with activation energies as low as 0.16 eV which means that sol-gel synthesized LATP is superior to other solid electrolytes. Our NMR results fully support recent theoretical investigations on the underlying diffusion mechanism, indicating that to rapidly jump from site to site, the ions use interstitial sites connected by low-energy barriers in LATP.
RESUMEN
The development of safe and long-lasting all-solid-state batteries with high energy density requires a thorough characterization of ion dynamics in solid electrolytes. Commonly, conductivity spectroscopy is used to study ion transport; much less frequently, however, atomic-scale methods such as nuclear magnetic resonance (NMR) are employed. Here, we studied long-range as well as short-range Li ion dynamics in the glass-ceramic Li7 P3 S11 . Li(+) diffusivity was probed by using a combination of different NMR techniques; the results are compared with those obtained from electrical conductivity measurements. Our NMR relaxometry data clearly reveal a very high Li(+) diffusivity, which is reflected in a so-called diffusion-induced (6) Li NMR spin-lattice relaxation peak showing up at temperatures as low as 313 K. At this temperature, the mean residence time between two successful Li jumps is in the order of 3×10(8) s(-1) , which corresponds to a Li(+) ion conductivity in the order of 10(-4) to 10(-3) S cm(-1) . Such a value is in perfect agreement with expectations for the crystalline but metastable glass ceramic Li7 P3 S11 . In contrast to conductivity measurements, NMR analysis reveals a range of activation energies with values ranging from 0.17 to 0.26 eV, characterizing Li diffusivity in the bulk. In our case, through-going Li ion transport, when probed by using macroscopic conductivity spectroscopy, however, seems to be influenced by blocking grain boundaries including, for example, amorphous regions surrounding the Li7 P3 S11 crystallites. As a result of this, long-range ion transport as seen by impedance spectroscopy is governed by an activation energy of approximately 0.38 eV. The findings emphasize how surface and grain boundary effects can drastically affect long-range ionic conduction. If we are to succeed in solid-state battery technology, such effects have to be brought under control by, for example, sophisticated densification or through the preparation of samples that are free of any amorphous regions that block fast ion transport.
RESUMEN
The introduction of structural disorder and large volume fractions of different kinds of interfaces enables the manipulation of ion dynamics in solids. Variable-temperature solid-state NMR relaxometry is highly useful to study Li(+) jump processes. If carried out as a function of frequency, the resulting NMR relaxation rates also contain information on the dimensionality (1D, 2D, or 3D) of the diffusion process. Recently, NMR relaxometry has revealed the 2D nature of Li hopping in LiBH4 , and thus this hydride is an interesting ion conductor for further diffusion studies on the spatially confined motion of Li spins. Here, nanocrystalline LiBH4 and the two-phase analogue LiBH4 :Al2 O3 , which are prepared by ball milling, serve as interesting model systems to track the changes in NMR relaxation rates with respect to coarse-grained, thermodynamically stable LiBH4 . This reveals that interface (nano)engineering influences the hexagonal-to-orthorhombic phase transition and thus alters the ion-transport properties of Li in one- and two-phase LiBH4 towards higher diffusivities at lower temperatures.
RESUMEN
The microscopic Li diffusion parameters in the lithiated spinel Li4 + xTi5O12, which is on its way to become a commercially used anode material in Li ion batteries, are probed for the first time via nuclear magnetic resonance spectroscopy.