Structural Modulation in the High Capacity Battery Cathode Material LiFeBO₃

The crystal structure of the promising Li-ion battery cathode material LiFeBO₃ 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 <i>a</i>-axis direction is observed. When the structure of LiFeBO₃ is refined in the 4-dimensional superspace group <i>C</i>2/<i>c</i>(α0γ)­00, with α = 1/2 and γ = 0 and with lattice parameters of <i>a</i> = 5.1681 Å, <i>b</i> = 8.8687 Å, <i>c</i> = 10.1656 Å, and β = 91.514°, all of the disorder present in the prior <i>C</i>2/<i>c</i> structural model is eliminated and a long-range ordering of 1D chains of corner-shared LiO₄ is revealed to occur as a result of cooperative displacements of Li and O atoms in the <i>c</i>-axis direction. Solid-state hybrid density functional theory calculations find that the modulation stabilizes the LiFeBO₃ structure by 1.2 kJ/mol (12 meV/f.u.), and that the modulation disappears after delithiation to form a structurally related FeBO₃ phase. The band gaps of LiFeBO₃ and FeBO₃ 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 LiFeBO₃ than in its unmodulated analogue

Structural Modulation in the High Capacity Battery Cathode Material LiFeBO₃

The crystal structure of the promising Li-ion battery cathode material LiFeBO₃ 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 <i>a</i>-axis direction is observed. When the structure of LiFeBO₃ is refined in the 4-dimensional superspace group <i>C</i>2/<i>c</i>(α0γ)­00, with α = 1/2 and γ = 0 and with lattice parameters of <i>a</i> = 5.1681 Å, <i>b</i> = 8.8687 Å, <i>c</i> = 10.1656 Å, and β = 91.514°, all of the disorder present in the prior <i>C</i>2/<i>c</i> structural model is eliminated and a long-range ordering of 1D chains of corner-shared LiO₄ is revealed to occur as a result of cooperative displacements of Li and O atoms in the <i>c</i>-axis direction. Solid-state hybrid density functional theory calculations find that the modulation stabilizes the LiFeBO₃ structure by 1.2 kJ/mol (12 meV/f.u.), and that the modulation disappears after delithiation to form a structurally related FeBO₃ phase. The band gaps of LiFeBO₃ and FeBO₃ 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 LiFeBO₃ than in its unmodulated analogue

Reciprocal Salt Flux Growth of LiFePO₄ Single Crystals with Controlled Defect Concentrations

Improved methods for the flux growth of single crystals of the important battery material LiFePO₄ have been developed, allowing the facile preparation of single crystals up to 1 cm across with well-developed facets at relatively low temperatures. The structural characterization of these samples by both powder X-ray diffraction and single crystal diffraction (X-ray and neutron) indicates that the samples are typically stoichiometric with a very low concentration of Fe defects on the Li site, though crystals with larger concentrations of defects can be specifically grown using Fe-rich fluxes. These defects occur through the formation of a Fe-rich (Li_{1–2<i>x</i>}Fe_<i>x</i>)­FePO₄ partial solid solution, in contrast to the antisite defects more commonly discussed in the literature which would preserve the ideal LiFePO₄ stoichiometry. The LiFePO₄ defects are shown to be sarcopside-like (2 Li⁺ → Fe²⁺ + vacancy) based on compositions refined from single crystal diffraction data, the observed dependence of unit cell parameters on defect concentration, and their observed phase behavior (defects only appear in growths from fluxes which are Fe-rich relative to stoichiometric LiFePO₄). The distribution of defects has been studied by aberration corrected scanning transmission electron microscopy and was found to be highly inhomogenous, suggesting that defect-containing crystals may consist of endotaxial intergrowths of olivine LiFePO₄ and sarcopside Fe₃(PO₄)₂ in a manner that minimizes the detrimental influence of Fe_Li defects on the rate of Li-ion transport within crystallites

Reciprocal Salt Flux Growth of LiFePO₄ Single Crystals with Controlled Defect Concentrations

Improved methods for the flux growth of single crystals of the important battery material LiFePO₄ have been developed, allowing the facile preparation of single crystals up to 1 cm across with well-developed facets at relatively low temperatures. The structural characterization of these samples by both powder X-ray diffraction and single crystal diffraction (X-ray and neutron) indicates that the samples are typically stoichiometric with a very low concentration of Fe defects on the Li site, though crystals with larger concentrations of defects can be specifically grown using Fe-rich fluxes. These defects occur through the formation of a Fe-rich (Li_{1–2<i>x</i>}Fe_<i>x</i>)­FePO₄ partial solid solution, in contrast to the antisite defects more commonly discussed in the literature which would preserve the ideal LiFePO₄ stoichiometry. The LiFePO₄ defects are shown to be sarcopside-like (2 Li⁺ → Fe²⁺ + vacancy) based on compositions refined from single crystal diffraction data, the observed dependence of unit cell parameters on defect concentration, and their observed phase behavior (defects only appear in growths from fluxes which are Fe-rich relative to stoichiometric LiFePO₄). The distribution of defects has been studied by aberration corrected scanning transmission electron microscopy and was found to be highly inhomogenous, suggesting that defect-containing crystals may consist of endotaxial intergrowths of olivine LiFePO₄ and sarcopside Fe₃(PO₄)₂ in a manner that minimizes the detrimental influence of Fe_Li defects on the rate of Li-ion transport within crystallites

Li₃Mo₄P₅O₂₄: A Two-Electron Cathode for Lithium-Ion Batteries with Three-Dimensional Diffusion Pathways

The structure of the novel compound Li₃Mo₄P₅O₂₄ has been solved from single crystal X-ray diffraction data. The Mo cations in Li₃Mo₄P₅O₂₄ are present in four distinct types of MoO₆ octahedra, each of which has one open vertex at the corner participating in a MoO double bond and whose other five corners are shared with PO₄ tetrahedra. On the basis of a bond valence sum difference map (BVS-DM) analysis, this framework is predicted to support the facile diffusion of Li⁺ ions, a hypothesis that is confirmed by electrochemical testing data, which show that Li₃Mo₄P₅O₂₄ can be utilized as a rechargeable battery cathode material. It is found that Li can both be removed from and inserted into Li₃Mo₄P₅O₂₄. The involvement of multiple redox processes occurring at the same Mo site is reflected in electrochemical plateaus around 3.8 V associated with the Mo⁶⁺/Mo⁵⁺ redox couple and 2.2 V associated with the Mo⁵⁺/Mo⁴⁺ redox couple. The two-electron redox properties of Mo cations in this structure lead to a theoretical capacity of 198 mAh/g. When cycled between 2.0 and 4.3 V versus Li⁺/Li, an initial capacity of 113 mAh/g is observed with 80% of this capacity retained over the first 20 cycles. This compound therefore represents a rare example of a solid state cathode able to support two-electron redox capacity and provides important general insights about pathways for designing next-generation cathodes with enhanced specific capacities

Ionic Conduction in Cubic Na₃TiP₃O₉N, a Secondary Na-Ion Battery Cathode with Extremely Low Volume Change

It is demonstrated that Na ions are mobile at room temperature in the nitridophosphate compound Na₃TiP₃O₉N, with a diffusion pathway that is calculated to be fully three-dimensional and isotropic. When used as a cathode in Na-ion batteries, Na₃TiP₃O₉N has an average voltage of 2.7 V vs Na⁺/Na and cycles with good reversibility through a mechanism that appears to be a single solid solution process without any intermediate plateaus. X-ray and neutron diffraction studies as well as first-principles calculations indicate that the volume change that occurs on Na-ion removal is only about 0.5%, a remarkably small volume change given the large ionic radius of Na⁺. Rietveld refinements indicate that the Na1 site is selectively depopulated during sodium removal. Furthermore, the refined displacement parameters support theoretical predictions that the lowest energy diffusion pathway incorporates the Na1 and Na3 sites while the Na2 site is relatively inaccessible. The measured room temperature ionic conductivity of Na₃TiP₃O₉N is substantial (4 × 10^–7 S/cm), though both the strong temperature dependence of Na-ion thermal parameters and the observed activation energy of 0.54 eV suggest that much higher ionic conductivities can be achieved with minimal heating. Excellent thermal stability is observed for both pristine Na₃TiP₃O₉N and desodiated Na₂TiP₃O₉N, suggesting that this phase can serve as a safe Na-ion battery electrode. Moreover, it is expected that further optimization of the general cubic framework of Na₃TiP₃O₉N by chemical substitution will result in thermostable solid state electrolytes with isotropic conductivities that can function at temperatures near or just above room temperature