Katja Weichert

Phase Boundary Propagation in Large LiFePO4 Single Crystals on Delithiation

Large single crystals of LiFePO(4) have been chemically delithiated. The relevance of chemical oxidation in comparison with electrochemical delithiation is discussed. Analyses of the Li content and profiles were done by electron energy loss spectroscopy and secondary ion mass spectrometry. The propagation of the FePO(4) phase growing on the surface of the large single crystal was followed by in situ optical microscopy as a function of time. The kinetics were evaluated in terms of linear irreversible thermodynamics and found to be characterized by an induction period followed by parabolic growth behavior of the FePO(4) phase indicating transport control. The growth rate was shown to depend on the crystallographic orientation. Scanning electron microscopy images showed cracks and a high porosity of the FePO(4) layer due to the significant changes in the molar volumes. The transport was found to be greatly enhanced by the porosity and crack formation and hence greatly enhanced over pure bulk transport, a result which is supposed to be very relevant for battery research if coarse-grained powder is used.

Delithiation study of LiFePO4 crystals using electron energy-loss spectroscopy

There is still some controversy about the mechanisms responsible for the delithiation of LiFePO 4 . We studied the delithiation of both large single crystals and powder by electron energy-loss spectroscopy. It is shown that Li depletion leads to a change in the Fe valency, to an enhanced prepeak at the O K edge, and to a pronounced interband transition peak between 4 and 6 eV. Exploiting these spectral changes, the delithiation process was monitored, with the result that in the large single crystal, delithiation takes place by removal of Li from the surface-near region, whereas in small particles, Li is removed from the particle core.

Atomistic Characterisation of Li+ Mobility and Conductivity in Li7−xPS6−xIx Argyrodites from Molecular Dynamics Simulations, Solid‐State NMR, and Impedance Spectroscopy

The atomistic mechanisms of Li(+) ion mobility/conductivity in Li(7-x)PS(6-x)I(x) argyrodites are explored from both experimental and theoretical viewpoints. Ionic conductivity in the title compound is associated with a solid-solid phase transition, which was characterised by low-temperature differential scanning calorimetry, (7)Li and (127)I NMR investigations, impedance measurements and molecular dynamics simulations. The NMR signals of both isotopes are dominated by anisotropic interactions at low temperatures. A significant narrowing of the NMR signal indicates a motional averaging of the anisotropic interactions above 177+/-2 K. The activation energy to ionic conductivity was assessed from both impedance spectroscopy and molecular dynamics simulations. The latter revealed that a series of interstitial sites become accessible to the Li(+) ions, whilst the remaining ions stay at their respective sites in the argyrodite lattice. The interstitial positions each correspond to the centres of tetrahedra of S/I atoms, and differ only in terms of their common corners, edges, or faces with adjacent PS(4) tetrahedra. From connectivity analyses and free-energy rankings, a specific tetrahedron is identified as the key restriction to ionic conductivity, and is clearly differentiated from local mobility, which follows a different mechanism with much lower activation energy. Interpolation of the lattice parameters as derived from X-ray diffraction experiments indicates a homogeneity range for Li(7-x)PS(6-x)I(x) with 0.97 < or = x < or = 1.00. Within this range, molecular dynamics simulations predict Li(+) conductivity at ambient conditions to vary considerably.

Lithium Potential Variations for Metastable Materials: Case Study of Nanocrystalline and Amorphous LiFePO4

Much attention has been paid to metastable materials in the lithium battery field, especially to nanocrystalline and amorphous materials. Nonetheless, fundamental issues such as lithium potential variations have not been pertinently addressed. Using LiFePO4 as a model system, we inspect such lithium potential variations for various lithium storage modes and evaluate them thermodynamically. The conclusions of this work are essential for an adequate understanding of the behavior of electrode materials and even helpful in the search for new energy materials.

In Situ Recording of Particle Network Formation in Liquids by Ion Conductivity Measurements

The formation of fractal silica networks from a colloidal initial state was followed in situ by ion conductivity measurements. The underlying effect is a high interfacial lithium ion conductivity arising when silica particles are brought into contact with Li salt-containing liquid electrolytes. The experimental results were modeled using Monte Carlo simulations and tested using confocal fluorescence laser microscopy and ζ-potential measurements.

LiFePO4: From an Insulator to a Robust Cathode Material

Ортофосфаты LiFe0,9M0,1PO4 со структурой оливина, допированные ванадием и титаном, были получены с помощью механохимически стимулированного твердофазного синтеза с использованием высокоэнергетической планетарной мельницы АГО-2 и последующего отжига при 750 °C. Показано, что ионы V и Ti не полностью замещают ионы Fe2+ в структуре LiFePO4. Оставшаяся часть этих ионов участвует в образовании второй фазы с насиконоподобной структурой: моноклинной Li3V2(PO4)3 (пространственная группа P21/n) и ромбоэдрической LiTi2(PO4)3 (пространственная группа R-3c). Согласно ПЭМ, средний размер частиц нанокомпозитов около 100–300 нм. ЭДС микроанализ показал, что мелкие частицы вторичных фаз сегрегированы на поверхности более крупных частиц LiFePO4. На зарядно-разрядных кривых LiFe0,9M0,1PO4 присутствуют плато, соответствующие LiFePO4 и второй фазе. Допирование ванадием повышает устойчивость циклирования LiFePO4 и улучшает его циклируемость при высоких скоростях в большей степени, чем в случае допирования титаном.