Shou-Hang Bo

Structural modulation in the high capacity battery cathode material LiFeBO3.

The crystal structure of the promising Li-ion battery cathode material LiFeBO(3) has been redetermined based on the results of single crystal X-ray diffraction data. A commensurate modulation that doubles the periodicity of the lattice in the a-axis direction is observed. When the structure of LiFeBO(3) is refined in the 4-dimensional superspace group C2/c(α0γ)00, with α = 1/2 and γ = 0 and with lattice parameters of a = 5.1681 Å, b = 8.8687 Å, c = 10.1656 Å, and β = 91.514°, all of the disorder present in the prior C2/c structural model is eliminated and a long-range ordering of 1D chains of corner-shared LiO(4) is revealed to occur as a result of cooperative displacements of Li and O atoms in the c-axis direction. Solid-state hybrid density functional theory calculations find that the modulation stabilizes the LiFeBO(3) structure by 1.2 kJ/mol (12 meV/f.u.), and that the modulation disappears after delithiation to form a structurally related FeBO(3) phase. The band gaps of LiFeBO(3) and FeBO(3) are calculated to be 3.5 and 3.3 eV, respectively. Bond valence sum maps have been used to identify and characterize the important Li conduction pathways, and suggest that the activation energies for Li diffusion will be higher in the modulated structure of LiFeBO(3) than in its unmodulated analogue.

High magnesium mobility in ternary spinel chalcogenides

Magnesium batteries appear a viable alternative to overcome the safety and energy density limitations faced by current lithium-ion technology. The development of a competitive magnesium battery is plagued by the existing notion of poor magnesium mobility in solids. Here we demonstrate by using ab initio calculations, nuclear magnetic resonance, and impedance spectroscopy measurements that substantial magnesium ion mobility can indeed be achieved in close-packed frameworks (~ 0.01–0.1 mS cm–1 at 298 K), specifically in the magnesium scandium selenide spinel. Our theoretical predictions also indicate that high magnesium ion mobility is possible in other chalcogenide spinels, opening the door for the realization of other magnesium solid ionic conductors and the eventual development of an all-solid-state magnesium battery.Low magnesium mobility in solids represents a significant obstacle to the development of Mg intercalation batteries. Here the authors show that substantial magnesium ion mobility can be achieved in close-packed ternary selenide spinel materials.

Influence of Inversion on Mg Mobility and Electrochemistry in Spinels

Magnesium oxide and sulfide spinels have recently attracted interest as cathode and electrolyte materials for energy-dense Mg batteries, but their observed electrochemical performance depends strongly on synthesis conditions. Using first-principles calculations and percolation theory, we explore the extent to which spinel inversion influences Mg2+ ionic mobility in MgMn2O4 as a prototypical cathode, and MgIn2S4 as a potential solid electrolyte. We find that spinel inversion and the resulting changes of the local cation ordering give rise to both increased and decreased Mg2+ migration barriers, along specific migration pathways, in the oxide as well as the sulfide. To quantify the impact of spinel inversion on macroscopic Mg2+ transport, we determine the percolation thresholds in both MgMn2O4 and MgIn2S4. Furthermore, we analyze the impact of inversion on the electrochemical properties of the MgMn2O4 cathode via changes in the phase behavior, average Mg insertion voltages and extractable capacities, at var...

In Situ Neutron Diffraction Studies of the Ion Exchange Synthesis Mechanism of Li2Mg2P3O9N: Evidence for a Hidden Phase Transition

Motivated by predictions made using a bond valence sum difference map (BVS-DM) analysis, the novel Li-ion conductor Li2Mg2P3O9N was synthesized by ion exchange from a Na2Mg2P3O9N precursor. Impedance spectroscopy measurements indicate that Li2Mg2P3O9N has a room temperature Li-ion conductivity of about 10-6 S/cm (comparable to LiPON), which is 6 orders of magnitude higher than the extrapolated Na-ion conductivity of Na2Mg2P3O9N at this temperature. The structure of Li2Mg2P3O9N was determined from ex situ synchrotron and time-of-flight neutron diffraction data to retain the P213 space group, though with a cubic lattice parameter of a = 9.11176(8) Å that is significantly smaller than the a = 9.2439(1) Å of Na2Mg2P3O9N. The two Li-ion sites are found to be very substantially displaced (∼0.5 Å) relative to the analogous Na sites in the precursor phase. The non-molten salt ion exchange method used to prepare Li2Mg2P3O9N produces a minimal background in powder diffraction experiments, and was therefore exploited for the first time to follow a Li+/Na+ ion exchange reaction using in situ powder neutron diffraction. Lattice parameter changes during ion exchange suggest that the reaction proceeds through a Na2-xLixMg2P3O9N solid solution (stage 1) followed by a two-phase reaction (stage 2) to form Li2Mg2P3O9N. However, full Rietveld refinements of the in situ neutron diffraction data indicate that the actual transformation mechanism is more complex and instead involves two thermodynamically distinct solid solutions in which the Li exclusively occupies the Li1 site at low Li contents (stage 1a) and then migrates to the Li3 site at higher Li contents (stage 1b), a crossover driven by the different signs of the local volume change at these sites. In addition to highlighting the importance of obtaining full structural data in situ throughout the ion exchange process, these results provide insights into the general question of what constitutes a thermodynamic phase.