Sodium-ion batteries are attractive energy storage media owing to the abundance of sodium, but the low capacities of available cathode materials make them impractical. Sodium-excess metal oxides Na2MO3 (M: transition metal) are appealing cathode materials that may realize large capacities through additional oxygen redox reaction. However, the general strategies for enhancing the capacity of Na2MO3 are poorly established. Here using two polymorphs of Na2RuO3, we demonstrate the critical role of honeycomb-type cation ordering in Na2MO3. Ordered Na2RuO3 with honeycomb-ordered [Na1/3Ru2/3]O2 slabs delivers a capacity of 180 mAh g−1 (1.3-electron reaction), whereas disordered Na2RuO3 only delivers 135 mAh g−1 (1.0-electron reaction). We clarify that the large extra capacity of ordered Na2RuO3 is enabled by a spontaneously ordered intermediate Na1RuO3 phase with ilmenite O1 structure, which induces frontier orbital reorganization to trigger the oxygen redox reaction, unveiling a general requisite for the stable oxygen redox reaction in high-capacity Na2MO3 cathodes.

Figure 6. a) Galvanostatic charge/discharge curves, and b) cycle stability of Na 2 Mn 3 O 7 between 1.5 and 4.7 V at a rate of C/20.

Reduction of thermal conductivity κ while preserving high electrical conductivity σ in materials continues to be a vital goal in thermoelectric study for the reuse of exhaust heat energy. In the use of an eco-friendly and ubiquitous element, Si as thermoelectric material, high κ value in bulk Si is the essential bottleneck to achieve high dimensionless figure of merit. This is a motivation for many recent studies on reducing κ in Si, by nanostructuring, e.g., using grains/wires with size smaller than the phonon mean free path. However, κ reduction that can be achieved tends to be saturated presumably due to an amorphous limit. Here, we present a nanoarchitecture for defeating the κ amorphous limit while preserving bulk-like σ. This new nanoarchitecture is an assembly of Si nanocrystals with oriented crystals separated by a 1-monolayer amorphous layer with well-controlled nanoscale shaped interfaces. At these interfaces, novel phonon scattering occurs resulting in κ reduction below the amorphous limit. Preservation of bulk-like σ results from the coherency of the carrier wavefunctions among the oriented nanocrystals separated by the ultrathin amorphous layer.

We have measured the growth rate of silicon nanowires (SiNWs), which were grown at temperatures between 365 and 495 °C via the vapor-liquid-solid (VLS) mechanism. We grew SiNWs using gold as catalysts and monosilane (SiH4) as a vapor phase reactant. Observing SiNWs by means of transmission electron microscopy, we have found that SiNWs with smaller diameters grow slower than those with larger ones, and the critical diameter at which growth stops completely exists. We have estimated the critical diameter of SiNWs to be about 2 nm. We have also measured the temperature dependence of the growth rate of SiNWs and estimated the activation energy of the growth of SiNWs to be 230kJ∕mol.

Reversibility of an electrode reaction is important for energy-efficient rechargeable batteries with a long battery life. Additional oxygen-redox reactions have become an intensive area of research to achieve a larger specific capacity of the positive electrode materials. However, most oxygen-redox electrodes exhibit a large voltage hysteresis >0.5 V upon charge/discharge, and hence possess unacceptably poor energy efficiency. The hysteresis is thought to originate from the formation of peroxide-like O22− dimers during the oxygen-redox reaction. Therefore, avoiding O-O dimer formation is an essential challenge to overcome. Here, we focus on Na2-xMn3O7, which we recently identified to exhibit a large reversible oxygen-redox capacity with an extremely small polarization of 0.04 V. Using spectroscopic and magnetic measurements, the existence of stable O−• was identified in Na2-xMn3O7. Computations reveal that O−• is thermodynamically favorable over the peroxide-like O22− dimer as a result of hole stabilization through a (σ + π) multiorbital Mn-O bond.