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Sodium-ion batteries are the next-generation in battery technology; however, their commercial development is hampered by electrode performance. The P2-type Na2/3(Fe½Mn½)O2 with a hexagonal structure and P63/mmc space group is considered a candidate sodium-ion battery cathode material due to its high capacity (∼190 m Ah·g-1) and energy density (∼520 m Wh·g-1), which are comparable to those of the commercial LiFePO4 and LiMn2O4 lithium-ion battery cathodes, with previously unexplained poor cycling performance being the major barrier to its commercial application. We use operando synchrotron X-ray powder diffraction to understand the origins of the capacity fade of the Na2/3(Fe½Mn½)O2 material during cycling over the relatively wide 1.5-4.2 V (vs Na) window. We found a complex phase-evolution, involving transitions from P63/mmc (P2-type at the open-circuit voltage) to P63 (OP4-type when fully charged) to P63/mmc (P2-type at 3.4-2.0 V) to Cmcm (P2-type at 2.0-1.5 V) symmetry structures during the desodiation and sodiation of the Na2/3(Fe½Mn½)O2 cathode. The associated large cell-volume changes with the multiple two-phase reactions are likely to be responsible for the poor cycling performance, clearly suggesting a 2.0-4.0 V window of operation as a strategy to improve cycling performance. We demonstrated here that the P2-type Na2/3(Fe½Mn½)O2 cathode is able to deliver ∼25% better cycling performance with the strategic operation window. This significant improvement in cycling performance implies that by characterizing the phase evolution and reaction mechanisms during battery function we are able to propose these modifications to the conditions of battery use that improve performance, highlighting the importance of the interplay between structure and electrochemistry. [ABSTRACT FROM AUTHOR]

Yeelimite, Ca4[Al6O12]SO4, is outstanding as an aluminatesodalite, being the framework ofthese type of materials flexible and dependent on ion sizes and anionordering/disordering. On the other hand, yeelimite is also importantfrom an applied perspective as it is the most important phase in calciumsulfoaluminate cements. However, its crystal structure is not wellstudied. Here, we characterize the room temperature crystal structureof stoichiometric yeelimite through joint Rietveld refinement usingneutron and X-ray powder diffraction data coupled with chemical soft-constraints.Our structural study shows that yeelimite has a lower symmetry thanthat of the previously reported tetragonal system, which we establishto likely be the acentric orthorhombic space group Pcc2, with a √2a × √2a × a superstructure basedon the cubic sodalite structure. Final unit cell values were a= 13.0356(7) Å, b= 13.0350(7) Å,and c= 9.1677(2) Å. We determine several structuresusing density functional theory calculations, with the lowest energystructure being Pcc2 in agreement with our experimentalresult. Yeelimite undergoes a reversible phase transition to a higher-symmetryphase which has been characterized to occur at 470 °C by thermodiffractometry.The higher-symmetry phase is likely cubic or pseudocubic possessingan incommensurate superstructure, as suggested by our theoreticalcalculations which show a phase transition from an orthorhombic toa tetragonal structure. Our theoretical study also predicts a pressure-inducedphase transition to a cubic structure of space group I43m. Finally, we show thatour reported crystal structure of yeelimite enables better mineralogicalphase analysis of commercial calcium sulfoaluminate cements, as shownby RFvalues for this phase, 6.9% and 4.8% for the previouslypublished orthorhombic structure and for the one reported in thisstudy, respectively. [ABSTRACT FROM AUTHOR]

Because of its stability, nanosized olivine LiFePO4 opens the door toward high-power Li-ion battery technology for large-scale applications as required for plug-in hybrid vehicles. Here, we reveal that the thermodynamics of first-order phase transitions in nanoinsertion materials is distinctly different from bulk materials as demonstrated by the decreasing miscibility gap that appears to be strongly dependent on the overall composition in LiFePO4. In contrast to our common thermodynamic knowledge, that dictates solubility limits to be independent of the overall composition, combined neutron and X-ray diffraction reveals strongly varying solubility limits below particle sizes of 35 nm. A rationale is found based on modeling of the diffuse interface. Size confinement of the lithium concentration gradient, which exists at the phase boundary, competes with the in bulk energetically favorable compositions. Consequently, temperature and size diagrams of nanomaterials require complete reconsideration, being strongly dependent on the overall composition. This is vital knowledge for the future nanoarchitecturing of superior energy storage devices as the performance will heavily depend on the disclosed nanoionic properties. [ABSTRACT FROM AUTHOR]